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Look up in the night sky and you’ll see lots of stars. But what is a star? In a scientific sense, a star is ball of hydrogen and helium with enough mass that it can sustain nuclear fusion at its core. Our Sun is a star, of course, but they can come in different sizes and colors. So let’s learn what a star is.
75% of the matter in the Universe is hydrogen and 23% is helium; these are the amounts left over from the Big Bang. These elements exist in large stable clouds of cold molecular gas. At some point a gravitational disturbance, like a supernova explosion or a galaxy collision will cause a cloud of gas to collapse, beginning the process of star formation.
As the gas collects together, it heats up. Conservation of momentum from the movement of all the particles in the cloud causes the whole cloud to begin spinning. Most of the mass collects in the center, but the rapid rotation of the cloud causes it to flatten out into a protoplanetary disk. It’s out of this disk that planets will eventually form, but that’s another story.
The protostar at the heart of the cloud heats up from the gravitational collapse of all the hydrogen and helium, and over the course of about 100,000 years, it gets hotter and hotter becoming a T Tauri star. Finally after about 100 million years of collapse, temperatures and pressures at its core become sufficient that nuclear fusion can ignite. From this point on, the object is a star.
Nuclear fusion is what defines a star, but they can vary in mass. And the different amounts of mass give a star its properties. The least massive star possible is about 75 times the mass of Jupiter. In other words, if you could find 74 more Jupiters and mash them together, you’d get a star. The most massive star possible is still an issue of scientific disagreement, but it’s thought to be about 150 times the mass of the Sun. More than that, and the star just can’t hold itself together.
The least massive stars are red dwarf stars, and will consume small amounts over tremendous periods of time. Astronomers have calculated that there are red dwarf stars that could live 10 trillion years. They put out a fraction of the energy released by the Sun. The largest supergiant stars, on the other hand, have very short lives. A star like Eta Carinae, with 150 times the mass of the Sun is emitting more than 1 million times as much energy as the Sun. It has probably only lasted a few million years and will soon detonate as a powerful supernova; destroying itself completely.
Most stars are in the main sequence phase of their lives, where they’re doing hydrogen fusion in their cores. Once this hydrogen runs out, and only helium is left in the core, the stars have to burn something else. The largest stars can continue fusing heavier and heavier elements until they can’t sustain fusion any more. The smallest stars eject their outer layers and become white dwarf stars, while the more massive stars have much more violent ends, become neutron stars and even black holes.
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Our Sun is a star. It’s a vast ball made up of 74% hydrogen and 24% helium, with trace amounts of other elements. It has so much mass that the temperatures and pressures at its core are hot enough to ignite fusion. At the core of the Sun (and other stars), atoms of hydrogen are being fused into atoms of helium. This process releases a tremendous amount of energy. If an object isn’t performing some kind of fusion at its core, it’s not a star.
Most planets are actually made of similar material to the Sun. Both Jupiter and Saturn have similar mixtures of hydrogen and helium. If the planet Jupiter is made of hydrogen, why doesn’t it shine like a star? It all comes down to mass. Jupiter would need to be about 80 times more massive before it had enough mass to actually ignite hydrogen fusion at its core.
The small rocky terrestrial planets like the Earth and Mars make up just a fraction of the mass of the Solar System. Unlike the larger gas giants, the terrestrial planets are mostly made up of denser elements, like iron, silicon and oxygen. The larger gas giant planets probably have large quantities of these heavier elements in their cores. In fact, Jupiter probably has an Earth-like ball of rock with 14 to 18 times the mass of the Earth at its core.
What about orbits? Planets orbit stars, no question. But you can also have multi-star systems where stars are orbiting stars. And it’s also possible that you could have binary planets orbiting a common center of gravity and together they orbit around a star.
The end of the day, the only real difference between planets and stars is mass – almost everything out there is made up of 75% hydrogen and 24% helium. If an object has about 80 times the mass of Jupiter, it has sufficient mass and temperature to ignite solar fusion in its core. If it doesn’t… it can’t
Would look twice as sweet! Are you seeing double? No. This isn’t an eye test – rather an incredible, dimensional look at NGC 2244 – a star cluster embroiled in a reflection nebula spanning 55 light-years and most commonly called “The Rosette.” Step inside and prepare to be blown away…
Do you remember the “magic eye” puzzles that were all the rage a few years ago? They were a series of meaningless spots until you relaxed your eyes, positioned the picture just the right distance and all at once… you could see dimension. This is exactly what will happen if you open this incredible full-sized image of the Rosette done by Jukka Metsavainio. It may take you a few moments to get your eyes in just the right position away from the monitor screen, but when you do? Wow… It’s like using a binocular viewer, but in living color!
Now, let’s learn about what we’re seeing…
Located about 2500 light-years away, the galactic star cluster NGC 2244 heats the gas within the nebula to nearly 18,000 degrees Fahrenheit, causing it to emit light in a process similar to that of a fluorescent tube. A huge percentage of this light is hydrogen-alpha, which is scattered back from its dusty shell and becomes polarized. The brightest and hottest of the stars that you see here are O type main sequence beauties – over a hundred times the size and a thousand times brighter than stars like our Sun. Their solar winds and radiation scream out, stripping the dust discs away from the younger stars and igniting the area in glowing florescence.
But deep inside, astronomers have discovered a young star coughing out a complex jet of material complete with knots and bow shocks. Thanks to the “O” boys clearing away the dusty debris, we’re able to hypothesize it may be a low-mass star, stripped of its accretion disc and left to evolve on its own. According to Zoltan Balog’s 2008 study; “Our observations support theoretical predictions in which photoevaporation removes the gas relatively quickly from the outer region of a protoplanetary disk, but leaves an inner, more robust, and possibly gas-rich disk component of radius 5-10 AU. With the gas gone, larger solid bodies in the outer disk can experience a high rate of collisions and produce elevated amounts of dust. This dust is being stripped from the system by the photon pressure of the O star to form a gas-free dusty tail.”
But that’s not all that’s going on inside this double rose… According to Junfeng Wang’s study with the Chandra X-Ray telescope; “By comparing the NGC 2244 and Orion Nebula Cluster, we estimate a total population of 2000 stars in NGC 2244. The spatial distribution of X-ray stars is strongly concentrated around the central O5 star, HD 46150. The other early O star, HD 46223, has few companions. The cluster’s stellar radial density profile shows two distinctive structures surrounded by an isothermal sphere extending out with core radius. This double structure, combined with the absence of mass segregation, indicates that this 2 million old cluster is not in dynamical equilibrium. The Rosette OB X-ray spectra are soft and consistent with the standard model of small-scale shocks in the inner wind of a single massive star.”
So what’s causing it? Possibly mass stellar segregation. While that seems more like a topic for a local newspaper than for an astronomy article, it’s true! According to the research done by L. Chen in 1977 who studied membership probabilities and velocity dispersions of stars in NGC 2244 it shows; “Clear evidence of mass segregation, but doesn’t exhibit any significant velocity-mass (or, equivalently, velocity-luminosity) dependence. This provides strong support for the suggestion that the observed mass segregation is at least partially due to the way in which star formation has proceeded in these complex star-forming regions (“primordial” mass segregation).” The effects of this internal two-body relaxation, may very well have simply come from NGC 2244 splitting apart a little sooner than expected! And what caused that? A strong probability of magnetic cluster stars…
While you won’t see any red hues in visible light, aim a large pair of binoculars about a fingerwidth east of Epsilon Monoceros (RA 6:32.4 Dec +04:52) from a dark sky site and see if you can make out a vague nebulosity associated with this open cluster. Even if you can’t, it is still a wonderful cluster of stars crowned by the yellow jewel of 12 Monocerotis. With good seeing, small telescopes can easily spot the broken, patchy wreath of nebulosity around a well-resolved symmetrical concentration of stars. Larger scopes, and those with filters, will make out separate areas of the nebula which also bear their own distinctive NGC labels. No matter how you view it, the entire region is one of the best for winter skies!
Eta Carinae Credit: Gemini Observatory artwork by Lynette Cook
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Since they’re all just made of hydrogen and helium, when it comes to stars, mass is everything. The amount of mass that a star has defines its luminosity, size and even how long it will live. The most massive stars in the Universe really live fast and die hard; they can amass more than 100 times the mass of the Sun, and will only live a few million years before detonating as supernovae.
How massive is massive? Some astronomers think that the theoretical limit for stellar mass is about 150 times the mass of the Sun (1 solar mass is the mass of the Sun), beyond this limit powerful stellar winds will push away infalling material before it can join the star. And stars with 150 solar masses have been observed, at least theoretically.
The most accurate way to measure the mass of an object like a star is if it’s in a binary system with another object. Astronomers can calculate the mass of the two objects by measuring how they orbit one another. But the most massive stars ever seen don’t have any binary companions, so astronomers have to guess at how massive they are. They estimate the star’s mass based on its temperature and absolute brightness.
There are dozens of known stars estimated to have 25 times the mass of the Sun. Here’s a list of the most massive known stars:
HD 269810 (150 solar masses)
Peony Nebula Star (150 solar masses)
Eta Carinae (150 solar masses)
Pistol Star (150 solar masses)
LBV 1806-20 (130 masses)
All of these stars are supergiant stars, which formed inside the largest clouds of gas and dust. Stars this large aren’t long for the Universe. They burn tremendous amounts of fuel and can be 500,000 times more luminous than the Sun.
Perhaps the most familiar, extremely massive star is Eta Carinae, located about 8,000 light years from Earth. Astronomers think it has an estimated mass of between 100 and 150 solar masses. The star is probably less than 3 million years old, and it’s believed that it has less than 100,000 years left to live. When it detonates, Eta Carinae’s supernova will be bright enough to see in the day, and you could read a book with it at night.
Dwarf Nova QZ Virginis - Annotated - Image Credit: Dr. Joe Brimacombe
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For all of you variable star fans, there’s a new kid on the block – Dwarf Nova QZ Virginis. It was originally discovered by T. Meshkova on Moscow photographic plates in 1944 and had a magnitude range of 12.9 to as little as 14.5 But what is it? Try a cataclysmic variable star – one that our good friends down under caught just for Universe Today readers!
According to recently released AAVSO Special Notice #144, dwarf nova QZ Vir (once known as T Leo) is currently in outburst, and it appears that this outburst is a supermaximum. Says M. Templeton, “The most recent visual estimate of QZ Vir puts the star at visual magnitude 10.2 (JD 2454857.6201; W. Kriebel, Walkenstetten, Germany). Time series photometry by W. Stein (New Mexico, United States) on 2009 Jan 25 indicates the presence of superhumps in the light curve. Observations by P. Schmeer (Saarburecken-Bischmisheim, Germany), E. Morelle (Lauwin-Planque, France), ASAS-3 (Pojmanski 2002, AcA52, 397) and R. Stubbings (Tetoora Road, Vic., Australia) published on VSNET. (T. Kato; vsnet-alert 10980) suggest QZ Vir may have had a short precursor outburst lasting 2-3 days and fading immediately before the rise to supermaximum. All observations, including both visual estimates and CCD time-series photometry, are strongly encouraged at this time.”
Of course, it didn’t take a lot of encouragement – only some clear skies to get astrophotographer and serious researcher Joe Brimacombe of Southern Galactic to set his telescope towards QZ Virginis and image for us. All we needed to do was provide the following coordinates:
RA: 11 38 26.80 , Dec: +03 22 07.0
Dwarf Nova QZ Virginis - Image Credit: Dr. Joe BrimacombeAs you can see, learning proper stellar coordinates is essential to practicing astronomy. Without them, a stellar field is simply a stellar field as it would be next to impossible to distinguish one background star from the next. While some of us understand what these strange sets of numbers mean – maybe some of our readers don’t. Let’s take just a moment out from our busy days and learn, shall we?
RA stands for Right Ascension. It is the celestial equivalent of terrestrial longitude. RA’s zero point is the Prime Meridian, located in the constellation of Aries where the Sun crosses the celestial equator at the March equinox. Each set of numbers is then measured eastward in three sets – hours, minutes, and seconds, with 24 hours being equivalent to a full circle. Declination, or “Dec” is comparable to latitude, projected onto the celestial sphere, and is measured in degrees north and south of the celestial equator. Points north of the celestial equator have positive declinations, while those to the south have negative declinations. These are also measured in three sets of numbers – degrees, minutes, and seconds of arc.
Now that you know, how do you use them? Chances are, if you have a telescope that has an equatorial mount, you already have the tools in your hands – called “setting circles”. These same sets of numbers are waiting right on your telescope for you to set them! Once your telescope is accurately polar aligned, you just use the setting circles to dial in these numbers and you’ll be right in the approximate area. For those with electronic setting circles, it’s just a matter of inputting the correct coordinates and comparing star fields. Once the general area is found, you simply need to understand how big the field your eyepiece gives and compare it to a star chart – like this one supplied by the AAVSO for QZ Vir.
AAVSO Locator Chart for QZ Vir
Make note of your observations and compare the suspect nova to other stars of known magnitude nearby. When you’re done – don’t keep your observations to yourself! Please report all observations to the AAVSO using the name “QZ Vir” and contribute!
A plaque attached to the side of the remains of pad 34. A solemn reminder of a black day in space history.
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As we go through our busy, every day lives, we scan the headlines in search of news. We pick up this story and that one, filing it away as part of who we are and what has happened in the world. Once in a great while we might take it back out and look at it again, but all too often we tend to forget as time goes on. Let’s change that today…
The era in which I grew up in worshipped astronauts as heroes. We didn’t see it as just another speciality job – or just another routine mission. These men, and eventually women, became larger than life. Human beings willing to take risks above and beyond the ordinary to expand our knowledge and our capabilities as a species. While we sit here comfy and cosy at our desks reading the daily space news, they orbit high above the Earth. Where we once took our daily drive to our factory jobs, they climbed inside experimental spacecraft. When the school bus drops our children off, the teachers go home to their every day lives, too. But not all of them, my friends…
Dave Reneke reminds us that the astronauts paid the ultimate price.
“As fate would have it, the tragedies that killed three Apollo astronauts and two space shuttle crews have anniversaries less than a week apart. Apollo 1 on January 27, 1967, Challenger on January 28, 1986, and Columbia on February 1, 2003. The first manned Apollo mission, Apollo 1, was scheduled for launch on 21 February 1967 at Cape Kennedy’s pad 34. Commander Gus Grissom, Ed White and Roger Chaffee were the flight crew. NASA, preparing for a future moon landing, knew this shakedown flight was a big step in that direction. Engineers, ground personnel and flight controllers were eager for this bird to fly.
Apollo 1 Crew. Ed White, Gus Grissom and Roger Chaffee.All checks had been made and confidence was high – however, Apollo 1 was an accident waiting to happen. A few weeks before launch the crew were 5 1/2 hours into a simulated countdown on 27 January 1967 at the Kennedy Space Centre when White cried, “Fire!” Chafee shouted, “We’re burning up.” In the oxygen-saturated cabin 70 metres in the air atop the Saturn IB rocket at Pad 34, White’s hand was seen trying to blow the hatch. It wouldn’t budge. “If White couldn’t get that hatch off, no one could,” astronaut Frank Borman said later.
Astronauts and their loved ones were in shock. Test pilots died while in the air, no one at NASA had prepared them for an accident on the ground. One of the original Mercury-7 astronauts of 1959, Grissom was 40 years old on the day of the Apollo 1 fire. White at 36 years of age had been pilot for the Gemini 4 mission during which he became the first American to walk in space. Selected as an astronaut in 1963, Chaffee was training for his first spaceflight. He was just 31 years of age.
An investigation later revealed major flaws in almost all aspects of the Apollo capsule’s design and construction. Investigators attributed a chafed wire underneath Grissom’s seat as sparking the inferno. With a great whoosh, like the sound of an oven being lit, the pure O2 in the cabin made every combustible item in the ship burn with super intensity. At the same time, no oxygen was left to breathe. The three astronauts were trapped in their melted suit material, fused with the charred nylon from the inside of the spacecraft. To remove the hatch, five rescuers struggled in thick smoke, each forced to make several trips in order to reach breathable air. Nothing could be done, it was simply too late!
Astronaut Frank Borman, a member of the investigating team, listened to the tape of his friends’ screams and felt himself becoming increasingly angrier with every cry for help he heard. Everywhere he and the rest of the investigation committee looked, they found sloppy workmanship by both the contractor and by NASA. Borman decided that he was going to do whatever it took to make sure the Apollo spacecraft flew again. And when it did, it would be the safest spacecraft ever built.
All that remains of the original Pad 34 complex where Ed White, Gus Grissom and Roger Chaffee lost their lives in a pad fire in 1967. Image credit Dave Reneke As a result, NASA abandoned the oxygen-rich atmosphere. More than 2,500 different items were removed and replaced with non-flammable materials. Engineers redesigned the hatch to open in 10 seconds compared to 90 seconds for the original. Borman, in his book ‘Countdown,’, described each NASA staff member who suffered depression, guilt or a breakdown as a “victim of Pad 34.” One NASA official drove onto a Houston expressway and raced his car at speeds of more than 160 kilometres an hour until the engine caught fire. Others dealt with it in their own way. The final ‘victim’ was White’s wife. She committed suicide in 1984.
NASA’s faster, better, cheaper policy had started to unravel, at the cost of human life – but a far more serious event was about to unfold as we built even bigger, more complex launch vehicles.
Space Shuttle Challenger seconds before it exploded killing all seven crew on board.The Space Shuttle Challenger Disaster took place on the morning of January 28, 1986, when Challenger broke apart 73 seconds into its flight. The New York Times declared the first space shuttle explosion the “worst disaster in space history.” It killed seven astronauts, including the first teacher in space, Christa McAuliffe. She was selected by NASA from more than 11,000 applicants and was scheduled to teach two lessons from Space Shuttle Challenger in orbit. McAuliffe’s third-grade son Scott along with her parents were just some of the thousands of people watching in wonder, then horror that morning as the ship blew apart high in the air.
Challenger Crew - The crew of STS-51-L: Front row from left, Mike Smith, Dick Scobee, Ron McNair. Back row from left, Ellison Onizuka, Christa McAuliffe, Greg Jarvis, Judith Resnik.Some believe the crew died instantly, others believe the capsule remained intact long enough as it was falling for them to realize their fate. We’ll never know. In the aftermath of the disaster, NASA was criticized for its lack of openness with the press. Shuttle flights were suspended pending an investigation, but NASA personnel still believed in the program and wanted it to continue. After a lengthy hiatus, Shuttles eventually flew again, but disaster was to strike one more time, and it came on the morning of February 1, 2003.
A single film frame of the Space Shuttle Columbia Breaking over Texas on February 1, 2003. The Space Shuttle Columbia disintegrated over Texas during re-entry into the Earth’s atmosphere, again killing all seven crew members. The loss of the spacecraft was a result of damage sustained during launch when a piece of foam insulation the size of a small briefcase hit the main propellant tank at launch, damaging the Shuttle’s tiles protecting it from the heat of re-entry. While Columbia was still in orbit, some engineers suspected damage, but NASA managers limited the investigation on the grounds that any risks were ‘acceptable.’
Coumbia Crew - On February 1, 2003, after a 16-day scientific mission, space shuttle Columbia disintegrated during its reentry into the Earth's atmosphere, killing astronauts Rick Husband, William McCool, Michael Anderson, David Brown, Kalpana Chawla, Laurel Clark, and the first Israeli astronaut in space, Ilan Ramon.Columbia was 16 minutes from home when the 2,500 degree heat of re-entry entered the cracked left hand wing and melted the aluminium struts. It exploded 70,000 metres over Texas. “The Columbia is lost. There are no survivors,” President George Bush told the nation.
Evelyn Husband giving a stirring speech at a remembrance ceremony at Kennedy Space Centre in February 2008. Image credit Dave RenekeOne year ago this week I flew to the USA and attended a memorial ceremony at the Kennedy Space Centre for the crew of Columbia. Among the invited guess was Evelyn Husband, wife of the shuttles’ Commander Rick Husband, who had previously piloted the first shuttle mission to dock with the International Space Station. In a stirring speech, and after all she’s been through, Evelyn expressed her earnest hope that the space program would go on. Let’s hope it does. This, they say, is the price of progress. ”
I would personally like to thank Dave Reneke for sharing his remembrance with us. As I sit here writing this story, I look around my office. Each and every wall bears a testimony of its own to the heroes of space – from pictures of mission launches and spacesuits – right down to a display of mission patches and model rockets. These heroes, be it Yuri Gagarin or Neil Armstrong, had a significant impact on my life and what I am today… Just as they may have had an impact on yours. Take the time to remember…
Neutron stars are formed when large stars run out of fuel and collapse. To get a neutron star, you need to have star that’s larger than about 1.5 solar masses and less than 5 times the mass of the Sun.
If you have less than 1.5 solar masses, you don’t have enough material and gravity to compress the object down enough. You only get a white dwarf. This is what will happen to our own Sun one day.
If you have more than 5 times the mass of the Sun, your star will end up as a black hole.
But if your star is right in between those masses, you get a neutron star.
The neutron star is formed when the star runs out of fuel and collapses inward on itself. The protons and electrons of atoms are forced together into neutrons. Since the star still has a lot of gravity, any additional material falling into the neutron star is super-accelerated by the gravity and turned into identical neutron material.
Just one teaspoon of a neutron star would have the mass of over 5 x 1012 kilograms.
A neutron star actually has different layers. Astronomers think there’s an outer shell of atomic nuclei with electrons about 1 meter thick. Below this crust, you get nuclei with increasing numbers of neutrons. These would decay quickly on Earth, but the intense pressure of the gravity keeps them stable.
When neutron stars form, they maintain the momentum of the entire star, but now they’re just a few kilometers across. This causes them to spin at tremendous rates, sometimes as fast as hundreds of times a second.
We have written many articles about stars on Universe Today. Here’s an article about a neutron star with a tail like a comet, and here’s an article about a a shooting star.
A shooting star is another name for a meteoroid that burns up as it passes through the Earth’s atmosphere. So, a shooting star isn’t a star at all.
Most of the shooting stars that we can see are known as meteoroids. These are objects as small as a piece of sand, and as large as a boulder. Smaller than a piece of sand, and astronomers call them interplanetary dust. If they’re larger than a boulder, astronomers call them asteroids.
A meteoroid becomes a meteor when it strikes the atmosphere and leaves a bright tail behind it. The bright line that we see in the sky is caused by the ram pressure of the meteoroid. It’s not actually caused by friction, as most people think.
When a meteoroid is larger, the streak in the sky is called a fireball or bolide. These can be bright, and leave a streak in the sky that can last for more than a minute. Some are so large they even make crackling noises as they pass through the atmosphere.
If any portion of the meteoroid actually survives its passage through the atmosphere, astronomers call them meteorites.
Some of the brightest and most popular meteor showers are the Leonids, the Geminids, and the Perseids. With some of these showers, you can see more than one meteor (or shooting star) each minute.
The term binary star is a misnomer because it is actually a star system made up of usually two stars that orbit around one center of mass – where the mass is most concentrated. A binary star is not to be confused with two stars that appear close together to the naked eye from Earth, but in reality are very far apart – Carl Sagan far!
Astrophysicists find binary systems to be quite useful in determining the mass of the individual stars involved. When two objects orbit one another, their mass can be calculated very precisely by using Newton’s calculations for gravity. The data collected from binary stars allows astrophysicists to extrapolate the relative mass of similar single stars.
There are several subcategories of binary stars, classified by their visual properties including eclipsing binaries, visual binaries, spectroscopic binaries and astrometric binaries.
Eclipsing binary stars are those whose orbits form a horizontal line from the point of observation; essentially, what the viewer sees is a double eclipse along a single plane; Algol for example.
A visual binary system is a system in which two separate stars are visible through a telescope that has an appropriate resolving power. These can be difficult to detect if one of the stars’ brightness is much greater, in effect blotting out the second star.
Spectroscopic binary stars are those systems in which the stars are very close and orbiting very quickly. These systems are determined by the presence of spectral lines – lines of color that are anomalies in an otherwise continuous spectrum and are one of the only ways of determining whether a second star is present. It is possible for a binary star system to be both a visual and a spectroscopic binary if the stars are far enough apart and the telescope being used is of a high enough resolution.
Astrometric binary stars are systems in which only one star can be observed, and the other’s presence is inferred by the noticeable wobble of the first star. This wobble happens as a result of the smaller star’s slight gravitational influence on the larger star.
So now you can answer the question, “what is a binary star?”
We have written many articles about binary stars on Universe Today. Here’s an article about a new class of binary stars discovered, and a situation where one star was ejected out of a binary partnership.
Were you wondering about the North Star? Firstly, you might expect one of the most famous stars in the night sky to be one of the brightest, but it isn’t; not by a long shot. That honor belongs to Sirius and many less bright stars besides. The North Star shines with a humble brightness that belies its navigational importance.
Polaris, or the North Star, sits almost directly above the North Pole; therefore, it is a reliable gauge of North if you find yourself lost on a clear night without a compass. Stars that sit directly above the Earth’s North or South Pole are called Pole Stars. Interestingly, the North Star hasn’t always been, nor will it always be the Pole Star because the Earth’s axis changes slightly over time, and stars move in relation to each other over time.
You can also approximate your latitude by measuring the angle of elevation between the horizon and the North Star. There is no equivalent star in the South Pole, but Sigma Octantis comes close. It isn’t very useful for navigational purposes as it isn’t very bright to the naked eye. Instead, navigators use two of the stars in the Southern Cross, Alpha and Gamma to determine due South.
The North Star is easy to find if you can first locate the Little Dipper. The North Star lies at the end of the handle in the Little Dipper (Ursa Minor). For a point of reference, The Big Dipper (Ursa Major) lies below the little dipper and their handles point in opposite directions. The two stars in the end of the ladle of The Big Dipper point to Polaris. Also, both The Big Dipper and The Little Dipper remain in the sky all night long, rotating in relation to the Earth’s axis.
