A montage of planets and other objects in the solar system. Credit: NASA/JPL
While the universe is a big place to study, we shouldn’t forget our own backyard. With eight planets and a wealth of smaller worlds to look at, there’s more than enough to learn for a few lifetimes!
So what are some of the most surprising things about the planets? We’ve highlighted a few things below.
1. Mercury is hot, but not too hot for ice
The closest planet to the Sun does indeed have ice on its surface. That sounds surprising at first glance, but the ice is found in permanently shadowed craters — those that never receive any sunlight. It is thought that perhaps comets delivered this ice to Mercury in the first place. In fact, NASA’s MESSENGER spacecraft not only found ice at the north pole, but it also found organics, which are the building blocks for life. Mercury is way too hot and airless for life as we know it, but it shows how these elements are distributed across the Solar System.
2. Venus doesn’t have any moons, and we aren’t sure why.
Both Mercury and Venus have no moons, which can be considered a surprise given there are dozens of other ones around the Solar System. Saturn has over 60, for example. And some moons are little more than captured asteroids, which may have been what happened with Mars’ two moons, for example. So what makes these planets different? No one is really sure why Venus doesn’t, but there is at least one stream of research that suggests it could have had one in the past.
Mars, as it appears today, Credit: NASA
3. Mars had a thicker atmosphere in the past.
What a bunch of contrasts in the inner Solar System: practically atmosphere-less Mercury, a runaway hothouse greenhouse effect happening in Venus’ thick atmosphere, temperate conditions on much of Earth and then a thin atmosphere on Mars. But look at the planet and you can see gullies carved in the past from probable water. Water requires more atmosphere, so Mars had more in the past. Where did it go? Some scientists believe it’s because the Sun’s energy pushed the lighter molecules out of Mars’ atmosphere over millions of years, decreasing the thickness over time.
4. Jupiter is a great comet catcher.
The most massive planet in the Solar System probably had a huge influence on its history. At 318 times the mass of Earth, you can imagine that any passing asteroid or comet going near Jupiter has a big chance of being caught or diverted. Maybe Jupiter was partly to blame for the great bombardment of small bodies that peppered our young Solar System early in its history, causing scars you can still see on the Moon today. And in 1994, astronomers worldwide were treated to a rare sight: a comet, Shoemaker-Levy 9, breaking up under Jupiter’s gravity and slamming into the atmosphere.
Fragmentation of comets is common. Many sungrazers are broken up by thermal and tidal stresses during their perihelions. At top, an image of the comet Shoemaker-Levy 9 (May 1994) after a close approach with Jupiter which tore the comet into numerous fragments. An image taken by Andrew Catsaitis of components B and C of Comet 73P/Schwassmann–Wachmann 3 as seen together on 31 May 2006 (Credit: NASA/HST, Wikipedia, A.Catsaitis)
5. No one knows how old Saturn’s rings are
There’s a field of ice and rock debris circling Saturn that from afar, appear as rings. Early telescope observations of the planet in the 1600s caused some confusion: does that planet have ears, or moons, or what? With better resolution, however, it soon became clear that there was a chain of small bodies encircling the gas giant. It’s possible that a single moon tore apart under Saturn’s strong gravity and produced the rings. Or, maybe they’ve been around (pun intended) for the last few billion years, unable to coalesce into a larger body but resistant enough to gravity not to break up.
6. Uranus is more stormy than we thought.
When Voyager 2 flew by the planet in the 1980s, scientists saw a mostly featureless blue ball and some assumed there wasn’t much activity going on on Uranus. We’ve had a better look at the data since then that does show some interesting movement in the southern hemisphere. Additionally, the planet drew closer to the Sun in 2007, and in more recent years telescope probing has shown some storms going on. What is causing all this activity is difficult to say unless we were to send another probe that way. And unfortunately, there are no missions yet that are slated for sure to zoom out to that part of the Solar System.
Infrared images of Uranus showing storms at 1.6 and 2.2 microns obtained Aug. 6, 2014 by the 10-meter Keck telescope. Credit: Imke de Pater (UC Berkeley) & Keck Observatory images.
7. Neptune has supersonic winds.
While on Earth we are concerned about hurricanes, the strength of these storms is nowhere near what you would find on Neptune. At its highest altitudes, according to NASA, winds blow at more than 1,100 miles per hour (1,770 kilometers per hour). To put that in context, that’s faster than the speed of sound on Earth, at sea level. Why Neptune is so blustery is a mystery, especially considering the Sun’s heat is so little at its distance.
8. You can see Earth’s magnetic field at work during light shows.
We have a magnetic field surrounding our planet that protects us from the blasts of radiation and particles the Sun sends our way. Good thing, too, because such flare-ups could prove deadly to unprotected people; that’s why NASA keeps an eye on solar activity for astronauts on the International Space Station, for example. At any rate, when you see auroras shining in the sky, that’s what happens when the particles from the Sun flow along the magnetic field lines and interact with Earth’s upper atmosphere.
Universe Today has many articles on interesting facts about the planets. Start with 10 facts about Mercury and 10 facts about Venus. You may also want to check out the 10 facts about Mars. Astronomy Cast also has a number of podcasts about the planets, including one on Earth.
Look at those astronauts, flying through space without a care in the world. But how can they be floating when there’s gravity pulling at them in every direction?
Hey look! It’s a montage of adorable astronauts engaging in hilarious space stuff in zero gravity. Look at them throwing bananas, playing Bowie songs, drinking floating juice balls, and generally having a gay old time in the weightlessness of deep space. It’s a camera inside a ball of water, you won’t believe what happens next! Or whatever it was they told you to get you to click that video.
Space isn’t all that far away, in fact, it’s likely closer than the next big city over. We have an equation to calculate gravitational pull between objects in space. It’s this little monster right here. It’s the “r” at the bottom we’re interested in here. When it’s a small value, like the short 370 km above your head there’s no remarkable difference between being on the space station or being on the surface. In fact, our beloved astronauts experience about 90% of the Earth’s gravity.
So why are they floating around so effortlessly in a most peculiar way? Shouldn’t they fall to the bottom of the space station? Shouldn’t the whole space station crash to the ground. Quickly, to the internet for our dramatic and creepy twilight zone style ending when we realize that the book was actually titled “How to cook forty humans!”. We have to tell someone!
According to our math those astronauts aren’t floating, they’re falling. THEY’RE FALLING.
And roll credits…So, the real twist was that NASA knew this all along. What looks like zero gravity is actually weightlessness. And you can get weightlessness whenever you’re falling.
You know that feeling when you crest a hill on a rollercoaster, or just as the elevator starts moving down? That’s you experiencing decreased weight. Jump out of an airplane, and you’ll experience seconds or even a minute of weightlessness before you have to open the chute. But the Earth moving towards you too rapidly for a little dirt-and-rock-cuddle-spooning time reminds you that this is falling, not flying.
Astronauts are orbiting Earth at a speed of 28,000 kilometers per hour, completing one spin around the planet every 90 minutes. As the astronauts accelerate towards our planet, the curvature of the Earth falls away from them – so they never actually slam into a horrible fiery twisted metal pancake of death.
Imagine there was a tower 370 km high. If you jumped off the top of the tower, you’d fall to the ground, near the base of the tower with a splat. Now, imagine if you jumped sideways off the tower. You might land a few kilometers away from the base of the tower. But still hit the ground. Now, imagine if you could run sideways at 28,000 km/h and you leap off the side of the tower. You’d still be falling, but the Earth is falling away at exactly the same rate, so you never actually hit the ground.
Despite years of training, many astronauts get motion sickness when they first arrive in orbit, and it can take a few days for them to become accustomed to the sensation.… And nobody judges them because they have the giant brass ones required to go into space in the first place.
Stephen Hawking, weightless (courtesy Zero Gravity Corporation)
NASA has developed a special aircraft to help astronauts get experience with weightlessness. It’s called the KC 135, it flies in the emperor of barfolpolis-inducing parabolas, and has the nickname “The Vomit Comet”. At the top of each parabola, the passengers of the KC 135 get to experience a few seconds of weightlessness before gravity catches up with them again and they fall down on the floor of the aircraft, followed with the experience of double gravity on the bottom of the parabola.
Then it’s upchuck city, or everyone takes a few moments to talk to ralph on the big white phone, or has a brief episode of the Technicolor-face-shouts-double-rainbarf across the sky.
What does it mean? What I’m saying is the vomit flows like a river.
In fact, there is no place you could go in the entire Universe where you could be in true zero gravity. Ever. At all. None. As we discussed in a previous episode, you’re under the influence of gravity of every single atom in the observable Universe. Without the Earth or the Sun here, you’d start falling into the center of the Milky Way. Or maybe into the Virgo Supercluster.
We’re all falling all the time. Fortunately we’re stuck to a giant ball which gives us a reference point where everything falls at the same rate we do including our atmosphere and lunch, both prior to and post consumption.
To best illustrate our point, I’m going to turn to Douglas Adams. He said in The Hitchhiker’s Guide to the Galaxy series: “the knack of flying is learning how to throw yourself at the ground and miss.” Do you want to experience true weightlessness? Would you be willing to go to orbit and give it a try?
A historical reconstruction of what Robert Hooke looked like. Credit: Wikipedia/Rita Greer/FAL
The spring is a marvel of human engineering and creativity. For one, it comes in so many varieties – the compression spring, the extension spring, the torsion spring, the coil spring, etc. – all of which serve different and specific functions. These functions in turn allow for the creation of many man-made objects, most of which emerged as part of the Scientific Revolution during the late 17th and 18th centuries.
As an elastic object used to store mechanical energy, the applications for them are extensive, making possible such things as an automotive suspension systems, pendulum clocks, hand sheers, wind-up toys, watches, rat traps, digital micromirror devices, and of course, the Slinky.
Like so many other devices invented over the centuries, a basic understanding of the mechanics is required before it can so widely used. In terms of springs, this means understanding the laws of elasticity, torsion and force that come into play – which together are known as Hooke’s Law.
Hooke’s Law is a principle of physics that states that the that the force needed to extend or compress a spring by some distance is proportional to that distance. The law is named after 17th century British physicist Robert Hooke, who sought to demonstrate the relationship between the forces applied to a spring and its elasticity.
Illustration of Hooke’s Law, showing the relationship between force and distance when applied to a spring. Credit: GSU/hyperphysics
He first stated the law in 1660 as a Latin anagram, and then published the solution in 1678 as ut tensio, sic vis – which translated, means “as the extension, so the force” or “the extension is proportional to the force”).
This can be expressed mathematically as F= -kX, where F is the force applied to the spring (either in the form of strain or stress); X is the displacement of the spring, with a negative value demonstrating that the displacement of the spring once it is stretched; and k is the spring constant and details just how stiff it is.
Hooke’s law is the first classical example of an explanation of elasticity – which is the property of an object or material which causes it to be restored to its original shape after distortion. This ability to return to a normal shape after experiencing distortion can be referred to as a “restoring force”. Understood in terms of Hooke’s Law, this restoring force is generally proportional to the amount of “stretch” experienced.
In addition to governing the behavior of springs, Hooke’s Law also applies in many other situations where an elastic body is deformed. These can include anything from inflating a balloon and pulling on a rubber band to measuring the amount of wind force is needed to make a tall building bend and sway.
This law has had many important practical applications, with one being the creation of a balance wheel, which made possible the creation of the mechanical clock, the portable timepiece, the spring scale and the manometer (aka. the pressure gauge). Also, because it is a close approximation of all solid bodies (as long as the forces of deformation are small enough), numerous branches of science and engineering as also indebted to Hooke for coming up with this law. These include the disciplines of seismology, molecular mechanics and acoustics.
However, like most classical mechanics, Hooke’s Law only works within a limited frame of reference. Because no material can be compressed beyond a certain minimum size (or stretched beyond a maximum size) without some permanent deformation or change of state, it only applies so long as a limited amount of force or deformation is involved. In fact, many materials will noticeably deviate from Hooke’s law well before those elastic limits are reached.
Still, in its general form, Hooke’s Law is compatible with Newton’s laws of static equilibrium. Together, they make it possible to deduce the relationship between strain and stress for complex objects in terms of the intrinsic materials of the properties it is made of. For example, one can deduce that a homogeneous rod with uniform cross section will behave like a simple spring when stretched, with a stiffness (k) directly proportional to its cross-section area and inversely proportional to its length.
The balance wheel in a cheap 1950s alarm clock (the Apollo, by Lux Mfg. Co.) showing the balance wheel (1) and regulator (2). Credit: Wikipedia/Public Domain
Another interesting thing about Hooke’s law is that it is a a perfect example of the First Law of Thermodynamics. Any spring when compressed or extended almost perfectly conserves the energy applied to it. The only energy lost is due to natural friction.
In addition, Hooke’s law contains within it a wave-like periodic function. A spring released from a deformed position will return to its original position with proportional force repeatedly in a periodic function. The wavelength and frequency of the motion can also be observed and calculated.
The modern theory of elasticity is a generalized variation on Hooke’s law, which states that the strain/deformation of an elastic object or material is proportional to the stress applied to it. However, since general stresses and strains may have multiple independent components, the “proportionality factor” may no longer be just a single real number.
A good example of this would be when dealing with wind, where the stress applied varies in intensity and direction. In cases like these, it is best to employ a linear map (aka. a tensor) that can be represented by a matrix of real numbers instead of a single value.
If you enjoyed this article there are several others that you will enjoy on Universe Today. Here is one about Sir Isaac Newton’s contributions to the many fields of science. Here is an interesting article about gravity.
There are also some great resources online, such as this lecture on Hooke’s Law that you can watch on academicearth.org. There is also a great explanation of elasticity on howstuffworks.com.
Mars is a constant point of discussion for space explorers around the world. We’ve sent dozens of spacecraft there to study it. Some want to land astronauts on it. The planet is just far away to make that dream difficult, but just close enough to spark our imagination. So what are some of the most important things to learn about the Red Planet?
Imagine the Big Bang, and you’re imagining an explosion. There must be come place we could travel in the Universe and see the wreckage left over from the Big Bang. So, where is it?
Close your eyes and imagine the Big Bang. That first moment, where all the energy, matter and light came into existence. It’s an explosion right? Fire, debris, sinks, marmots and anvils flying past the camera in an ever expanding cloud of hot gas.
And like any explosion, there must be an aftermath, right? Some place we could travel in the Universe and see the exact spot that everything began; the exact location where the Big Bang happened and ideally a huge crater in spacetime where the Universe began.
I expect you’re imagining our little scene in your mind. Complete with space-time indentations and orbital detritus. I hope you’re also getting the unsettling feeling of dread that I’m about to smash up beloved sci-fi tropes for my own amusement. And here it is…
There’s no exact spot that the Big Bang happened. In fact, the Big Bang happened everywhere in the Universe. The problem generally comes from the term “Big Bang”. It brings to mind explosions, detonations, balloons being popped, and everything being blown out to chickenbasket hades. It’s too bad for us regular folk, this isn’t a good descriptive term for what the Big Bang was.
So I’m going to propose a new term, and just use it from here on out, and pretend like it was always this way. So, from here on out, I’m going to call it the Big Stretch, and by that I mean I’ve always called it the Big Stretch, and for those of you familiar with this type of retconning, the chocolate ration is being increased from 40 grams to 25 grams.
Imagine a balloon covered in dots, then inflate the balloon. Also, for the purposes of this illustration, you’re a 2-dimensional creature living at one of those dots and watching all the other dots. From your perspective, everything will smell like that weird damp spit and rubber balloon scent.
You’ll also see all other other dots moving away from you. You might even think you’re at the center of the expansion of the balloon. And then if you jumped to any other dot, you’d see the same thing. Just smelly dots, all racing away from you.
Expansion of the Universe. Image credit: Eugenio Bianchi, Carlo Rovelli & Rocky Kolb.
Now a lesser being would get all caught up thinking about the fact that the balloon is a three-dimensional object, and the center of the expansion is actually at the middle of the balloon. But you’re a 2D creature. You can’t comprehend anything but the surface of the balloon. That and the funky smell.
Now take that concept and scale it up one more dimension. As a three-dimensional creature trapped within a three-dimensional Universe witnessing it stretching out three dimensions. Every galaxy is moving away from you. But if you travel to any other galaxy, it looks like all the other galaxies are moving away from them.
Could a four-dimensional being find the center of the expansion, the place where the Big Bang happened? Probably. 4D beings are cool like that. But then, a 5D being would probably laugh at their simplistic 4D view of the Universe, with their quaint Klein bottles and rustic hypercubes. Suck it 4D jerks, they’d say, and then they’d trap them in their 5D lockers for the entirety of recess until the janitor heard the banging and let them out.
And don’t get me started on those 11D jerks. Those guys are awful, and they really think they’re better than everyone else. They’re like Greg Marmand from Omega House but with 8 more dimensions of nose to look down at you across.
So, where did the Big Bang happen? It happened everywhere. All places formed in the Big Bang – I mean – Big Stretch, and they’ve been moving away from each other for 13.8 billion years. There’s no one place you can point to and say: the Big Bang happened there.But you can be totally obnoxious and point to anywhere, and say the Big Bang happened there. Since the Big Bang happened everywhere, it happened in your hometown. Tell us where you’re from in the comments below.
Betelgeuse was the first star directly imaged -- besides our own Sun, of course. Image obtained by the Hubble Space Telescope. Credit: Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA
While there are untold billions of celestial objects visible in the nighttime sky, some of them are better known than others. Most of these are stars that are visible to the naked eye and very bright compared to other stellar objects. For this reason, most of them have a long history of being observed and studied by human beings, and most likely occupy an important place in ancient folklore.
So without further ado, here is a sampling of some of the better-known stars in that are visible in the nighttime sky:
Polaris: Also known as the North Star (as well as the Pole Star, Lodestar, and sometimes Guiding Star), Polaris is the 45th brightest star in the night sky. It is very close to the north celestial pole, which is why it has been used as a navigational tool in the northern hemisphere for centuries. Scientifically speaking, this star is known as Alpha Ursae Minoris because it is the alpha star in the constellation Ursa Minor (the Little Bear).
The Polaris star system, as seen within the Ursa Minor constellation and up close. Credit: NASA, ESA, N. Evans (Harvard-Smithsonian CfA), and H. Bond (STScI)
It’s more than 430 light-years away from Earth, but its luminosity (being a white supergiant) makes it highly visible to us here on Earth. What’s more, rather than being a single supergiant, Polaris is actually a trinary star system, comprised of a main star (alpha UMi Aa) and two smaller companions (alpha UMi B, alpha UMi Ab). These, along with its two distant components (alpha UMi C, alpha UMi D), make it a multistar system.
Interestingly enough, Polaris wasn’t always the north star. That’s because Earth’s axis wobbles over thousands of years and points in different directions. But until such time as Earth’s axis moves farther away from the “Polestar”, it remains our guide.
Because it is what is known as a Cepheid variable star – i.e. a star that pulsates radially, varying in both temperature and diameter to produce brightness changes – it’s distance to our Sun has been the subject of revision. Many scientific papers suggest that it may be up to 30% closer to our Solar System than previously expected – putting it in the vicinity of 238 light years away.
Time exposure centered on Polaris, the North Star. Notice that the closer stars are to Polaris, the smaller the circles they describe. Stars at the edge of the frame make much larger circles. Credit: Bob King
Sirius: Also known as the Dog Star, because it’s the brightest star in Canis Major (the “Big Dog”), Sirius is also the brightest star in the night sky. The name “Sirius” is derived from the Ancient Greek “Seirios“, which translates to “glowing” or “scorcher”. Whereas it appears to be a single bright star to the naked eye, Sirius is actually a binary star system, consisting of a white main-sequence star named Sirius A, and a faint white dwarf companion named Sirius B.
The reason why it is so bright in the sky is due to a combination of its luminosity and distance – at 6.8 light years, it is one of Earth’s nearest neighbors. And in truth, it is actually getting closer. For the next 60,000 years or so, astronomers expect that it will continue to approach our Solar System; at which point, it will begin to recede again.
In ancient Egypt, it was seen as a signal that the flooding of the Nile was close at hand. For the Greeks, the rising of Sirius in the night sky was a sign of the”dog days of summer”. To the Polynesians in the southern hemisphere, it marked the approach of winter and was an important star for navigation around the Pacific Ocean.
Alpha Centauri System: Also known as Rigel Kent or Toliman, Alpha Centauri is the brightest star in the southern constellation of Centaurus and the third brightest star in the night sky. It is also the closest star system to Earth, at just a shade over four light-years. But much like Sirius and Polaris, it is actually a multistar system, consisting of Alpha Centauri A, B, and Proxima Centauri (aka. Centauri C).
Artist’s impression of the planet around Alpha Centauri B. Credit: ESO
Based on their spectral classifications, Alpha Centauri A is a main sequence white dwarf with roughly 110% of the mass and 151.9% the luminosity of our Sun. Alpha Centauri B is an orange subgiant with 90.7% of the Sun’s mass and 44.5% of its luminosity. Proxima Centauri, the smallest of the three, is a red dwarf roughly 0.12 times the mass of our Sun, and which is the closest of the three to our Solar System.
English explorer Robert Hues was the first European to make a recorded mention of Alpha Centauri, which he did in his 1592 work Tractatus de Globis. In 1689, Jesuit priest and astronomer Jean Richaud confirmed the existence of a second star in the system. Proxima Centauri was discovered in 1915 by Scottish astronomer Robert Innes, Director of the Union Observatory in Johannesburg, South Africa.
Betelgeuse: Pronounced “Beetle-juice” (yes, the same as the 1988 Tim Burton movie), this bright red supergiant is roughly 65o light-year from Earth. Also known as Alpha Orionis, it is nevertheless easy to spot in the Orion constellation since it is one of the largest and most luminous stars in the night sky.
Betelgeuse, as seen by the Hubble Space Telescope, and in relation to the Orion constellation. Credit: NASA
The star’s name is derived from the Arabic name Ibt al-Jauza’, which literally means “the hand of Orion”. In 1985, Margarita Karovska and colleagues from the Harvard–Smithsonian Center for Astrophysics, announced the discovery of two close companions orbiting Betelgeuse. While this remains unconfirmed, the existence of possible companions remains an intriguing possibility.
What excites astronomers about Betelgeuse is it will one day go supernova, which is sure to be a spectacular event that people on Earth will be able to see. However, the exact date of when that might happen remains unknown.
Rigel: Also known as Beta Orionis, and located between 700 and 900 light years away, Rigel is the brightest star in the constellation Orion and the seventh brightest star in the night sky. Here too, what appears to be a blue supergiant is actually a multistar system. The primary star (Rigel A) is a blue-white supergiant that is 21 times more massive than our sun, and shines with approximately 120,000 times the luminosity.
Rigel B is itself a binary system, consisting of two main sequence blue-white subdwarf stars. Rigel B is the more massive of the pair, weighing in at 2.5 Solar masses versus Rigel C’s 1.9. Rigel has been recognized as being a binary since at least 1831 when German astronomer F.G.W. Struve first measured it. A fourth star in the system has been proposed, but it is generally considered that this is a misinterpretation of the main star’s variability.
Rigel A is a young star, being only 10 million years old. And given its size, it is expected to go supernova when it reaches the end of its life.
Vega: Vega is another bright blue star that anchors the otherwise faint Lyra constellation (the Harp). Along with Deneb (from Cygnus) and Altair (from Aquila), it is a part of the Summer Triangle in the Northern hemisphere. It is also the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere (after Arcturus).
Characterized as a white dwarf star, Vega is roughly 2.1 times as massive as our Sun. Together with Arcturus and Sirius, it is one of the most luminous stars in the Sun’s neighborhood. It is a relatively close star at only 25 light-years from Earth.
Vega was the first star other than the Sun to be photographed and the first to have its spectrum recorded. It was also one of the first stars whose distance was estimated through parallax measurements, and has served as the baseline for calibrating the photometric brightness scale. Vega’s extensive history of study has led it to be termed “arguably the next most important star in the sky after the Sun.”
Artist’s concept of a recent massive collision of dwarf planet-sized objects that may have contributed to the dust ring around the star Vega. Credit: NASA/JPL/Caltech/T. Pyle (SSC)
Based on observations that showed excess emission of infrared radiation, Vega is believed to have a circumstellar disk of dust. This dust is likely to be the result of collisions between objects in an orbiting debris disk. For this reason, stars that display an infrared excess because of circumstellar dust are termed “Vega-like stars”.
Thousands of years ago, (ca. 12,000 BCE) Vega was used as the North Star is today, and will be so again around the year 13,727 CE.
Pleiades: Also known as the “Seven Sisters”, Messier 45 or M45, Pleiades is actually an open star cluster located in the constellation of Taurus. At an average distance of 444 light years from our Sun, it is one of the nearest star clusters to Earth, and the most visible to the naked eye. Though the seven largest stars are the most apparent, the cluster actually consists of over 1,000 confirmed members (along with several unconfirmed binaries).
The core radius of the cluster is about 8 light years across, while it measures some 43 light years at the outer edges. It is dominated by young, hot blue stars, though brown dwarfs – which are just a fraction of the Sun’s mass – are believed to account for 25% of its member stars.
Pleiades, also known as M45, is a prominent open star cluster in the sky. Image Credit: Jamie Ball
The age of the cluster has been estimated at between 75 and 150 million years, and it is slowly moving in the direction of the “feet” of what is currently the constellation of Orion. The cluster has had several meanings for many different cultures here on Earth, which include representations in Biblical, ancient Greek, Asian, and traditional Native American folklore.
Antares: Also known as Alpha Scorpii, Antares is a red supergiant and one of the largest and most luminous observable stars in the nighttime sky. It’s name – which is Greek for “rival to Mars” (aka. Ares) – refers to its reddish appearance, which resembles Mars in some respects. It’s location is also close to the ecliptic, the imaginary band in the sky where the planets, Moon and Sun move.
This supergiant is estimated to be 17 times more massive, 850 times larger in terms of diameter, and 10,000 times more luminous than our Sun. Hence why it can be seen with the naked eye, despite being approximately 550 light-years from Earth. The most recent estimates place its age at 12 million years.
A red supergiant, Antares is over 850 times the diameter of our own Sun, 15 times more massive, and 10,000 times brighter. Credit: NASA/Ivan Eder
Antares is the seventeenth brightest star that can be seen with the naked eye and the brightest star in the constellation Scorpius. Along with Aldebaran, Regulus, and Fomalhaut, Antares comprises the group known as the ‘Royal stars of Persia’ – four stars that the ancient Persians (circa. 3000 BCE) believed guarded the four districts of the heavens.
Canopus: Also known as Alpha Carinae, this white giant is the brightest star in the southern constellation of Carina and the second brightest star in the nighttime sky. Located over 300 light-years away from Earth, this star is named after the mythological Canopus, the navigator for king Menelaus of Sparta in The Iliad.
Thought it was not visible to the ancient Greeks and Romans, the star was known to the ancient Egyptians, as well as the Navajo, Chinese and ancient Indo-Aryan people. In Vedic literature, Canopus is associated with Agastya, a revered sage who is believed to have lived during the 6th or 7th century BCE. To the Chinese, Canopus was known as the “Star of the Old Man”, and was charted by astronomer Yi Xing in 724 CE.
Image of Canopus, as taken by crew members aboard the ISS. Credit: NASA
It is also referred to by its Arabic name Suhayl (Soheil in persian), which was given to it by Islamic scholars in the 7th Century CE. To the Bedouin people of the Negev and Sinai, it was also known as Suhayl, and used along with Polaris as the two principal stars for navigation at night.
It was not until 1592 that it was brought to the attention of European observers, once again by Robert Hues who recorded his observations of it alongside Achernar and Alpha Centauri in his Tractatus de Globis (1592).
As he noted of these three stars, “Now, therefore, there are but three Stars of the first magnitude that I could perceive in all those parts which are never seene here in England. The first of these is that bright Star in the sterne of Argo which they call Canobus. The second is in the end of Eridanus. The third is in the right foote of the Centaure.”
The Milky Way is a spiral galaxy with several prominent arms containing stellar nurseries swathed in pink clouds of hydrogen gas. The sun is shown near the bottom in the Orion Spur. Credit: NASA
We live in a galaxy that is called the Milky Way. It’s called a barred spiral galaxy, which means that it has a spiral shape with a bar of stars across its middle. The galaxy is rather huge — at least 100,000 light-years in diameter, making it the second-biggest in our Local Group of galaxies.
More mind-blowing is that this mass of stars, gas, planets and other objects are all spinning. Just like a pinwheel. It’s spinning at 270 kilometers per second (168 miles per second) and takes about 200 million years to complete one rotation, according to the National Radio Astronomy Observatory. But why? More details below.
It’s worth taking a quick detour to talk about how long it takes the Solar System to move around the center of the galaxy. According to National Geographic, that’s about 225 million years. Dinosaurs were starting to arise the last time we were in the position we are today.
The rising Milky Way at Sentosa Island in Singapore. Credit and copyright: Justin Ng.
Scientists have mapped the spin using the Very Large Baseline Array, a set of radio telescopes. They examined spots where stars were forming and paid particular attention to those areas where gas molecules enhance radio emission, according to the National Radio Astronomy Observatory. Dubbed “cosmic masers”, these areas shine brightly in radio waves.
As Earth moves in its orbit, the shift of these molecules can be mapped against more distant objects. Measuring this shift shows how the entire galaxy rotates — and can even provide information about the mass of the Milky Way. So that’s all very neat, but why is it rotating in the first place?
If we think back to the early Universe, there are two big assumptions astronomers make, according to How Stuff Works: there was a lot of hydrogen and helium, with some parts denser than other areas. In the denser areas, gas clumped together in protogalactic clouds; the thickest areas collapsed into stars.
In this image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile young stars huddle together against a backdrop of clouds of glowing gas and lanes of dust. The star cluster, known as NGC 3293, would have been just a cloud of gas and dust itself about ten million years ago, but as stars began to form it became the bright group we see here. Clusters like this are celestial laboratories that allow astronomers to learn more about how stars evolve. Image Credit: ESO/G. Beccari
“These stars burned out quickly and became globular clusters, but gravity continued to collapse the clouds,” How Stuff Works wrote. “As the clouds collapsed, they formed rotating disks. The rotating disks attracted more gas and dust with gravity and formed galactic disks. Inside the galactic disk, new stars formed. What remained on the outskirts of the original cloud were globular clusters and the halo composed of gas, dust and dark matter.”
A simpler way to think about this is if you’re creating a pizza by tossing a ball of dough into the air. The spin of the dough creates a flat disc — just like what you observe in more complicated form in the Milky Way, not to mention other galaxies.
Artists illustration of the expansion of the Universe (Credit: NASA, Goddard Space Flight Center)
The Universe is expanding, but how quickly is it expanding? How far away is everything getting from everything else? And how do we know any of this anyway?
When astronomers talk about the expansion of the Universe, they usually express it in terms of the Hubble parameter. First introduced by Edwin Hubble when he demonstrated that more distant galaxies are moving away from us faster than closer ones.The best measurements for this parameter gives a value of about 68 km/s per megaparsec.
Let’s recap. Hubble. Universe. Galaxies. Leaving. Further means faster. And then I said something that sounded like “blah blah Lando blah blah Kessel Run 68 km/s per megaparsec”. Which translates to if you have a galaxy 1 megaparsec away, that’s 3.3 million light years for those of you who haven’t seen Star Wars, it would be expanding away from us at a speed of 68 km/s. So, 1 megaparsec in distance means it’s racing away at 68 km/s.
This is all because space is expanding everywhere in all places, and as a result distant galaxies appear to be expanding away from us faster than closer ones. There’s just more “space” to expand between us and them in the first place. Even better, our Universe was much more dense in the past, as a result the Hubble parameter hasn’t always had the same value.
There are two things affecting the Hubble parameter: dark energy, working to drive the Universe outwards, and matter, dark and regular flavor trying to hold it together. Pro tip: The matter side of this fight is currently losing.
Expansion of the Universe. Image credit: Eugenio Bianchi, Carlo Rovelli & Rocky Kolb.
Earlier in the Universe, when the Hubble parameter was smaller, matter had a stronger influence due to its higher overall density. Today dark energy is dominant, thus the Hubble parameter is larger, and this is why we talk about the Universe not only expanding but accelerating.
Our cosmos expands at about the rate at which space is expanding, and the speed at which objects expand away from us depends upon their distance. If you go far enough out, there is a distance at which objects are speeding away from us faster than the speed of light. As a result, it’s suspected that receding galaxies will cross a type of cosmological event horizon, where any evidence of their existence, not even light, would ever be able to reach us, no matter how far into the future you went.
What do you think? Is there anything out there past that cosmological event horizon line waiting to surprise us?
Star trails over Lake Minnewanka in Alberta, Canada. Credit and copyright: Jack-Fusco.
We know that Earth is not the center of the universe — let alone the Solar System — but looking at the sky, it’s easy to get confused. Stars appear to be rising and setting, as well as the planets, Moon and the Sun. And with more precise instruments, we can see some stars appearing to move back and forth relative to other ones.
As we’ll see below, we can explain those movements through the Earth’s rotation and movement through its orbit. But stars also have their own proper motion through space. So when we say that stars “move”, it could be because of the Earth, because of their own movements, or because of both!
The Earth takes roughly 24 hours to spin on its axis, moving from east to west. And if you watch the sky over a few hours in most locations on Earth, you can see the same thing happening: stars rising in the east, and setting in the west. There are some exceptions to this rule, however:
Star Trails by Cory Schmitz
Stars that are close to the Earth’s axis of rotation — what we call the north and the south pole — rotate around the poles. If the pole’s location is far enough above the horizon, some stars never set. They just keep spinning.
If your geographical location happens to be close to the pole, most stars will be rotating around the pole and very few will rise and set. (And in a trick of geometry, it will be hard to see the Sun, moon and planets since their path in the sky is at 23.5 degrees — the same as Earth’s tilt. This is why the poles have months of darkness, because the Sun doesn’t always shine there.)
So we’ve covered the Earth’s rotation, but we’ve neglected to mention its orbit around the Sun. It takes us about 365 days to make a full trip. As we move along in space, some curious effects occur. Consider the famous Mars mystery; astronomers used to be puzzled as to why the planet appeared to stop its movement against the background stars, go backwards and then go forwards again. Turns out it was Earth in its orbit “catching up” to the more distant Mars and passing it by.
Global mosaic of Mars showing the dark basaltic Syrtis Major Planus region made from Viking Orbiter images. (NSSDC)
At opposite ends of our orbit — say, in winter and summer — we can even see some stars appearing to shift against the background. If you picture the Earth in its orbit around the Sun, recall that we orbit about 93 million miles (150 million kilometers) from our closest neighbor. So at opposite ends of the orbit, Earth’s position is double that — 186 million miles (300 million kilometers).
Here’s where it gets interesting. Imagine you’re doing laps around a baseball field, looking at a building about a mile (1.6 kilometers) away. That building will appear to shift positions as you move around the track. The same thing happens when the Earth moves around in its orbit. Some of the closer stars can be seen moving back and forth across the background. We call this effect parallax and we can use it for stars that are as far away as about 100 light-years. We can actually calculate their distance using some geometry.
With parallax technique, astronomers observe object at opposite ends of Earth’s orbit around the Sun to precisely measure its distance. CREDIT: Alexandra Angelich, NRAO/AUI/NSF.
So we’ve covered ways the stars “move” due to the Earth’s orbit. But stars can move for other reasons as well. Maybe we’re observing a binary system where two stars are orbiting around each other. Maybe the stars are embedded in a galaxy that is itself rotating. Maybe the star is moving due to the expansion of the Universe, which gradually stretches distances between objects.
But stars also have their own motion in space — called proper motion — that is independent of these phenomena. Why is the star moving? Simply put, it’s because of gravity — because they are moving around the center of their galaxy, for example. Gravity makes every object in space move. But as most stars are far away from us and space is so big, that proper motion is very small in a human lifetime. The star with the highest proper motion is Barnard’s Star. It moves 10.3 seconds of arc per year, meaning it takes about 180 years for it to move the diameter of the full Moon in our sky.
The Sun provides energy for life here on Earth through light and heat. Credit: NASA Goddard Space Flight Center
A quick think about optical astronomy would have you imagine that most of it takes place at night. Isn’t that when the stars and galaxies come out to play? Well, that assumption makes at least one glaring error: Earth happens to be close to a star that is worthy of study. It’s called the Sun, and it only appears during the day.
We love being close to the Sun because it gives energy that gives us light. But that same energy can also be damaging to eyes and to instruments. Below are how amateurs and professionals alike safely observe our closest stellar neighbor.
Amateur astronomy
The safest way to observe the Sun is by projecting it on to a surface. By doing this, you’ll be able to see huge sunspots and you can also watch as the star marches through a solar eclipse — if you’re lucky enough to be in the area.
This is how Sky & Telescope suggests you get it done: “Poke a small hole in an index card with a pencil point, face it toward the Sun, and hold a second card three or four feet behind it in its shadow. The hole will project a small image of the Sun’s disk onto the lower card.”
The partial solar eclipse on Nov. 2, 2013 at its peak over Israel. Credit and copyright: Gadi Eidelheit.
If you prefer to look at the Sun directly, you must protect your eyes and your equipment (binoculars/telescope/camera) from looking at it unexposed. We’ll refer you again to the Sky & Telescope article for the best expertise, but in general, understand that you will need special equipment to do it safely.
Professional astronomy
There are numerous larger telescopes that are used on the ground, which typically have special filters to block out the damaging parts of the Sun’s light. We have a few examples below, but we’re sure you’ll come up with more examples from your own neighborhoods!
Of note, professional astronomers use multiple tools to look at the Sun. They can examine the Sun in different wavelengths of light to see its surface and corona. They can use spectroscopy to see the elements produced in different parts of the Sun. They can study its radiation using radar, or its interior using techniques such as acoustic interferometry.
U.S. National Solar Observatory: The observatory has two major optical facilities, called the Dunn Solar Telescope (Sacramento Peak) and the McMath-Pierce Solar Telescope (Kitt Peak). Luckily for the public, both are open to visitors. The observatory also is part of the Global Oscillation Network Group, which looks at acoustic waves inside the Sun using six stations spaced around the world.
Big Bear Solar Observatory‘s New Solar Telescope can view features on the Sun that are as small as 50 miles (80 kilometers) across. It saw “first light” in 2010 and for now, is the largest aperture solar telescope at 1.6 meters across.
But that’s not all we’ve got. Here are a few examples of space telescopes in orbit:
The Sun as viewed by the Solar and Heliospheric Observatory (NASA/SOHO)
Solar and Heliospheric Observatory (SOHO): Launched in 1995, this NASA and European Space Agency is supposed to study the Sun’s interior, figure out more about the superheated solar corona or envelope that surrounds the Sun, and understand how the solar wind is created. It’s also a famous comet catcher and observer.
STEREO (Solar TErrestrial RElations Observatory):Launched in 2006, these twin spacecraft are in different parts of the Earth’s orbit: one ahead, and one behind. Their goal is to produce three-dimensional images of the Sun to improve space weather forecasting (specifically, when large eruptions on the Sun could disrupt Earth communications). As of early 2015, STEREO-B is not communicating with Earth.
Solar Dynamics Observatory: Launched in 2010, it aims to understand why the Sun has an 11-year solar cycle and to learn more about the Sun’s magnetic field and energy. The ultimate goal, again, is to improve space weather predictions.
We have written many articles about solar observatories, both ground and space-based, here on Universe Today. Here’s an article about the STEREO spacecraft seeing a tsunami on the Sun. We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.
