Stars Archives

August 2, 2013December 23, 2015 by David Dickinson

The Astronomy of the Dog Days of Summer

Looking east from latitude 30 north on August 3rd, 30 minutes before sunrise. (Created by the author in Stellarium).

Can you feel the heat?

It’s not just your imagination. The northern hemisphere is currently in the midst of the Dog Days of Summer. For many, early August means hot, humid days and stagnant, sultry nights.

The actual dates for the Dog Days of Summer vary depending on the source, but are usually quoted as running from mid-July to mid-August. The Old Farmer’s Almanac lists the Dog Days as running from July 3^rd through August 11^th.

But there is an ancient astronomical observation that ties in with the Dog Days of Summer, one that you can replicate on these early August mornings.

The sky was important to the ancients. It told them when seasons were approaching, when to plant crops, and when to harvest. Ancient cultures were keen observers of the cycles in the sky. Cultures that were “astronomically literate” had a distinct edge over those who seldom bothered to note the goings on overhead.

The flooded Temple of Isis on the island of Philae circa 1905. (Credit: Wikimedia Commons under an Attribution-Share Alike 2.5 license. Author H.W. Dunning).

Sirius was a key star for Egyptian astronomers. Identified with the goddess Isis, the Egyptian name for Sirius was Sopdet, the deification of Sothis. There is a line penned by the Greco-Roman scholar Plutarch which states:

“The soul of Isis is called ‘Dog’ by the Greeks.”

Political commentary? A mis-translation by Greek scholars? Whatever the case, the mythological transition from “Isis to Sothis to Dog Star” seems to have been lost in time.

These astronomer-priests noted that Sirius rose with the Sun just prior to the annual flooding of the Nile. The appearance of a celestial object at sunrise is known as a heliacal rising. If you can recover Sirius from behind the glare of the Sun, you know that the “Tears of Isis” are on their way, in the form of life-giving flood waters.

Sopdet as the personification of Sirius (note the star on the forehead) Wikimedia Commons image under an Attribution Share Alike 3.0 license. Author Jeff Dahl).

In fact, the ancient Egyptians based their calendar on the appearance of Sirius and what is known as the Sothic cycle, which is a span of 1,461 sidereal years (365.25 x 4) in which the heliacal rising once again “syncs up” with the solar calendar.

It’s interesting to note that in 3000 BC, the heliacal rising of Sirius and the flooding of the Nile occurred around June 25^th, near the summer solstice. This also marked the Egyptian New Year. Today it occurs within a few weeks of August 15^th, owing to precession. (More on that in a bit!)

By the time of the Greeks, we start to see Sirius firmly referred to as the Dog Star. In Homer’s Iliad, King Priam refers to an advancing Achilles as:

“Blazing as the star that cometh forth at Harvest-time, shining forth amid the host of stars in the darkness of the night, the star whose name men call Orion’s Dog”

The Romans further promoted the canine branding for Sirius. You also see references to the “Dog Star” popping up in Virgil’s Aenid.

Over the years, scholars have also attempted to link the dog-headed god Anubis to Sirius. This transition is debated by scholars, and in his Star Names: Their Lore and Meaning, Richard Hinckley Allen casts doubt on the assertion.

Sirius as the shining "nose" of the constellation Canis Major. (Created by the author using Starry Night). — Sirius as the shining “nose” of the constellation Canis Major. (Created by the author using Starry Night).

Ancient cultures also saw the appearance of Sirius as signifying the onset of epidemics. Their fears were well founded, as summer flooding would also hatch a fresh wave of malaria and dengue fever-carrying mosquitoes.

Making a seasonal sighting of Sirius is fun and easy to do. The star is currently low to the southeast in the dawn, and rises successively higher each morning as August rolls on.

The following table can be used to aid your quest in Sirius-spotting.

Latitude north	Theoretical date when Sirius can 1^st be spotted
32°	August 3^rd
33°	August 4^th
34°	August 5^th
35°	August 6^th
36°	August 7^th
37°	August 8^th
38°	August 9^th
39°	August 10^th
40°	August 11^th
41°	August 12^th
42°	August 13^th
43°	August 14^th
44°	August 15^th
45°	August 16^th
46°	August 17^th
47°	August 18^th
48°	August 19^th
49°	August 20^th
50°	August 21^st

Thanks to “human astronomical computer extraordinaire” Ed Kotapish for the compilation!

Note that the table above is perpetual for years in the first half of the 21^st century. Our friend, the Precession of the Equinoxes pivots the equinoctial points to the tune of about one degree every 72 years. The Earth’s axis completes one full “wobble” approximately every 26,000 years. Our rotational pole only happens to be currently pointing at Polaris in our lifetimes. Its closest approach is around 2100 AD, after which the north celestial pole and Polaris will begin to drift apart. Mark your calendars—Vega will be the pole star in 13,727 AD. And to the ancient Egyptians, Thuban in the constellation Draco was the Pole Star!

Near Luxor (Photo by author). — The Colossi of Memnon Near Luxor, just one of the amazing architectural projects carried out by the ancient Egyptians. (Photo by author).

Keep in mind, atmospheric extinction is your enemy in this quest, as it will knock normally brilliant magnitude -1.46 Sirius a whopping 40 times in brightness to around magnitude +2.4.

Note that we have a nice line-up of planets in the dawn sky (see intro chart), which are joined by a waning crescent Moon this weekend. Jupiter and Mars ride high about an hour before sunrise, and if you can pick out Mercury at magnitude -0.5 directly below them, you should have a shot at spotting Sirius far to the south.

And don’t be afraid to “cheat” a little bit and use binoculars in your quest… we’ve even managed on occasion to track Sirius into the broad daylight. Just be sure to physically block the Sun behind a building or hill before attempting this feat!

Sirius as seen via Hubble- can you spy Sirius B? (NASA/ESA Hubble image). — Sirius as seen via Hubble- can you spy Sirius B? (Credit: NASA/ESA Hubble image).

Of course, the heliacal rising of Sirius prior to the flooding of the Nile was a convenient coincidence that the Egyptians used to their advantage. The ancients had little idea as to what they were seeing. At 8.6 light-years distant, Sirius is the brightest star in Earth’s sky during the current epoch. It’s also the second closest star visible to the naked eye from Earth. Only Alpha Centauri, located deep in the southern hemisphere sky is closer. The light you’re seeing from Sirius today left in early 2005, back before most of us had Facebook accounts.

Sirius also has a companion star, Sirius B. This star is the closest example of a white dwarf. Orbiting its primary once every 50 years, Sirius B has also been the center of a strange controversy we’ve explored in past writings concerning Dogon people of Mali.

Sirius B is difficult to nab in a telescope, owing to dazzling nearby Sirius A. This feat will get easier as Sirius B approaches apastron with a max separation of 11.5 arc seconds in 2025.

Some paleoastronomers have also puzzled over ancient records referring to Sirius as “red” in color. While some have stated that this might overturn current astrophysical models, a far more likely explanation is its position low to the horizon for northern hemisphere observers. Many bright stars can take on a twinkling ruddy hue when seen low in the sky due to atmospheric distortion.

Let the Dog Days of Summer (& astronomy) begin! (Photo by author).

All great facts to ponder during these Dog Days of early August, perhaps as the sky brightens during the dawn and your vigil for the Perseid meteors draws to an end!

July 29, 2013December 23, 2015 by Elizabeth Howell

Astronomers See Snow … In Space!

Artist's conception of the snow line in TW Hydrae. Credit: Bill Saxton/Alexandra Angelich, NRAO/AUI/NSF

There’s an excellent chance of frost in this corner of the universe: astronomers have spotted a “snow line” in a baby solar system about 175 light-years away from Earth. The find is cool (literally and figuratively) in itself. More importantly, however, it could give us clues about how our own planet formed billions of years ago.

“[This] is extremely exciting because of what it tells us about the very early period in the history of our own solar system,” stated Chunhua Qi, a researcher with the Harvard-Smithsonian Center for Astrophysics who led the research.

“We can now see previously hidden details about the frozen outer reaches of another solar system, one that has much in common with our own when it was less than 10 million years old,” he added.

The real deal enhanced-color picture of TW Hydrae is below, courtesy of a newly completed telescope: the Atacama Large Millimeter/submillimeter Array in Chile. It is designed to look at grains and other debris around forming solar systems. This snow line is huge, stretching far beyond the equivalent orbit of Neptune in our own solar system. See the circle? That’s Neptune’s orbit. The green stuff is the snow line. Look just how far the green goes past the orbit.

The carbon monoxide line as seen by the Atacama Large Millimeter/submillimeter Array (ALMA) telescope. The circle represents the equivalent orbit of Neptune when comparing it to our own solar system. Credit: Karin Oberg, Harvard University/University of Virginia — The carbon monoxide line on TW Hydrae as seen by the Atacama Large Millimeter/submillimeter Array (ALMA) telescope. The circle represents the equivalent orbit of Neptune when comparing it to our own solar system. Credit: Karin Oberg, Harvard University/University of Virginia

Young stars are typically surrounded by a cloud of gas and debris that, astronomers believe, can in many cases form into planets given enough time. Snow lines form in young solar systems in areas where the heat of the star isn’t enough to melt the substance. Water is the first substance to freeze around dust grains, followed by carbon dioxide, methane and carbon monoxide.

It’s hard to spot them: “Snow lines form exclusively in the relatively narrow central plane of a protoplanetary disk. Above and below this region, stellar radiation keeps the gases warm, preventing them from forming ice,” the astronomers stated. In areas where dust and gas are more dense, the substances are insulated and can freeze — but it’s difficult to see the snow through the gas.

In this case, astronomers were able to spot the carbon monoxide snow because they looked for diazenylium, a molecule that is broken up in areas of carbon monoxide gas. Spotting it is a “proxy” for spots where the CO froze out, the astronomers said.

Here are some more of the many reasons this is exciting to astronomers:

Snow could help dust grains form faster into rocks and eventually, planets because it coats the grain surface into something more stickable;
Carbon monoxide is a requirement to create methanol, considered a building block of complex molecules and life;
The snow was actually spotted with only a small portion of ALMA’s 66 antennas while it was still under construction. Now that ALMA is complete, scientists are already eager to see what the telescope will turn up the next time it gazes at the system.

Source: National Radio Astronomy Observatory

July 17, 2013December 23, 2015 by Jason Major

Earth’s Gold Came From Colliding Stars

Collisions of neutron stars produce powerful gamma-ray bursts – and heavy elements like gold (Credit: Dana Berry, SkyWorks Digital, Inc.)

Are you wearing a gold ring? Or perhaps gold-plated earrings? Maybe you have some gold fillings in your teeth… for that matter, the human body itself naturally contains gold — 0.000014%, to be exact! But regardless of where and how much of the precious yellow metal you may have with you at this very moment, it all ultimately came from the same place.

And no, I don’t mean Fort Knox, the jewelry store, or even under the ground — all the gold on Earth likely originated from violent collisions between neutron stars, billions of years in the past.

Recent research by scientists at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts has revealed that considerable amounts of gold — along with other heavy elements — are produced during impacts between neutron stars, the super-dense remains of stars originally 1.4 to 9 times the mass of our Sun.

The team’s investigation of a short-duration gamma-ray outburst that occurred in June (GRB 130603B) showed a surprising residual near-infrared glow, possibly from a cloud of material created during the stellar merger. This cloud is thought to contain a considerable amount of freshly-minted heavy elements, including gold.

“We estimate that the amount of gold produced and ejected during the merger of the two neutron stars may be as large as 10 moon masses – quite a lot of bling!” said lead author Edo Berger.

"With this remnant of a dead neutron star, I thee wed." (FreeDigitalPhotos.net/bigjom) — “With this remnant of a dead neutron star, I thee wed.” (FreeDigitalPhotos.net/bigjom)

The mass of the Moon is 7.347 x 10²² kg… about 1.2% the mass of Earth. The collision between these neutron stars then, 3.9 billion light-years away, produced 10 times that much gold based on the team’s estimates.

Quite a lot of bling, indeed.

Gamma-ray bursts come in two varieties – long and short – depending on the duration of the gamma-ray flash. GRB 130603B, detected by NASA’s Swift satellite on June 3rd, lasted for less than two-tenths of a second.

Although the gamma rays disappeared quickly, GRB 130603B also displayed a slowly fading glow dominated by infrared light. Its brightness and behavior didn’t match the typical “afterglow” created when a high-speed jet of particles slams into the surrounding environment.

Instead, the glow behaved like it came from exotic radioactive elements. The neutron-rich material ejected by colliding neutron stars can generate such elements, which then undergo radioactive decay, emitting a glow that’s dominated by infrared light – exactly what the team observed.

“We’ve been looking for a ‘smoking gun’ to link a short gamma-ray burst with a neutron star collision,” said Wen-fai Fong, a graduate student at CfA and a co-author of the paper. “The radioactive glow from GRB 130603B may be that smoking gun.”

The team calculates that about one-hundredth of a solar mass of material was ejected by the gamma-ray burst, some of which was gold. By combining the estimated gold produced by a single short GRB with the number of such explosions that have likely occurred over the entire age of the Universe, all the gold in the cosmos – and thus on Earth – may very well have come from such gamma-ray bursts.

Watch an animation of two colliding neutron stars along with the resulting GRB below (Credit: Dana Berry, SkyWorks Digital, Inc.):

How much gold is there on Earth, by the way? Since most of it lies deep inside Earth’s core and is thus unreachable, the total amount ever retrieved by humans over the course of history is surprisingly small: about 172,000 tonnes, or enough to make a cube 20.7 meters (68 feet) per side (based on the Thomson Reuters GFMS annual survey.) Some other estimates put this amount at slightly more or less, but the bottom line is that there really isn’t all that much gold available in Earth’s crust… which is partly what makes it (and other “precious” metals) so valuable.

And perhaps the knowledge that every single ounce of that gold was created by dead stars smashing together billions of years ago in some distant part of the Universe would add to that value.

“To paraphrase Carl Sagan, we are all star stuff, and our jewelry is colliding-star stuff,” Berger said.

The team’s findings were presented today in a press conference at the CfA in Cambridge. (See the paper here.)

Source: Harvard-Smithsonian CfA

July 17, 2013December 23, 2015 by David Dickinson

Water-Trapped Worlds Possible Around Red Dwarf Stars?

An artist's concept of a rocky world orbiting a red dwarf star. (Credit: NASA/D. Aguilar/Harvard-Smithsonian center for Astrophysics).

Hunters of alien life may have a new and unsuspected niche to scout out.

A recent paper submitted by Associate Professor of Astronomy at Columbia University Kristen Menou to the Astrophysical Journal suggests that tidally-locked planets in close orbits to M-class red dwarf stars may host a very unique hydrological cycle. And in some extreme cases, that cycle may cause a curious dichotomy, with ice collecting on the farside hemisphere of the world, leaving a parched sunward side. Life sprouting up in such conditions would be a challenge, experts say, but it is — enticingly — conceivable.

The possibility of life around red dwarf stars has tantalized researchers before. M-type dwarfs are only 0.075 to 0.6 times as massive as our Sun, and are much more common in the universe. The life span of these miserly stars can be measured in the trillions of years for the low end of the mass scale. For comparison, the Universe has only been around for 13.8 billion years. This is another plus in the game of giving biological life a chance to get underway. And while the habitable zone, or the “Goldilocks” region where water would remain liquid is closer in to a host star for a planet orbiting a red dwarf, it is also more extensive than what we inhabit in our own solar system.

Gliese 581- an example of a potential habitable zone around a red dwarf star contrasted with our own solar system. (Credit: ESO/Henrykus under a Wikimedia Creative Commons Attribution 3.0 Unported license).

But such a scenario isn’t without its drawbacks. Red dwarfs are turbulent stars, unleashing radiation storms that would render any nearby planets sterile for life as we know it.

But the model Professor Menou proposes paints a unique and compelling picture. While water on the permanent daytime side of a terrestrial-sized world tidally locked in orbit around an M-dwarf star would quickly evaporate, it would be transported by atmospheric convection and freeze out and accumulate on the permanent nighttime side. This ice would only slowly migrate back to the scorching daytime side and the process would continue.

Could these types of “water-locked worlds” be more common than our own?

The type of tidal locking referred to is the same as has occurred between the Earth and its Moon. The Moon keeps one face eternally turned towards the Earth, completing one revolution every 29.5 day synodic period. We also see this same phenomenon in the satellites for Jupiter and Saturn, and such behavior is most likely common in the realm of exoplanets closely orbiting their host stars.

The study used a dynamical model known as PlanetSimulator created at the University of Hamburg in Germany. The worlds modeled by the author suggest that planets with less than a quarter of the water present in the Earth’s oceans and subject to a similar insolation as Earth from its host star would eventually trap most of their water as ice on the planet’s night side.

Kepler data results suggest that planets in close orbits around M-dwarf stars may be relatively common. The author also notes that such an ice-trap on a water-deficient world orbiting an M-dwarf star would have a profound effect of the climate, dependent on the amount of volatiles available. This includes the possibility of impacts on the process of erosion, weathering, and CO₂ cycling which are also crucial to life as we know it on Earth.

Thus far, there is yet to be a true “short list” of discovered exoplanets that may fit the bill. “Any planet in the habitable zone of an M-dwarf star is a potential water-trapped world, though probably not if we know the planet possesses a thick atmosphere.” Professor Menou told Universe Today. “But as more such planets are discovered, there should be many more potential candidates.”

Hard times in harsh climes-an artist's conception of the daytime side of a world orbiting a red dwarf star. — Hard times in harsh climes-an artist’s conception of the daytime side of a world orbiting a red dwarf star. (Credit: NASA/JPL-Caltech).

Being that red dwarf stars are relatively common, could this ice-trap scenario be widespread as well?

“In short, yes,” Professor Menou said to Universe Today. “It also depends on the frequency of planets around such stars (indications suggest it is high) and on the total amount of water at the surface of the planet, which some formation models suggest should indeed be small, which would make this scenario more likely/relevant. It could, in principle, be the norm rather than the exception, although it remains to be seen.”

Of course, life under such conditions would face the unique challenges. The daytime side of the world would be subject to the tempestuous whims of its red dwarf host sun in the form of frequent radiation storms. The cold nighttime side would offer some respite from this, but finding a reliable source of energy on the permanently shrouded night side of such as world would be difficult, perhaps relying on chemosynthesis instead of solar-powered photosynthesis.

On Earth, life situated near “black smokers” or volcanic vents deep on the ocean floor where the Sun never shines do just that. One could also perhaps imagine life that finds a niche in the twilight regions of such a world, feeding on the detritus that circulates by.

Some of the closest red dwarf stars to our own solar system include Promixa Centauri, Barnard’s Star and Luyten’s Flare Star. Barnard’s star has been the target of searches for exoplanets for over a century due to its high proper motion, which have so far turned up naught.

The closest M-dwarf star with exoplanets discovered thus far is Gliese 674, at 14.8 light years distant. The current tally of extrasolar worlds as per the Extrasolar Planet Encyclopedia stands at 919.

This hunt will also provide a challenge for TESS, the Transiting Exoplanet Survey Satellite and the successor to Kepler due to launch in 2017.

Searching for and identifying ice-trapped worlds may prove to be a challenge. Such planets would exhibit a contrast in albedo, or brightness from one hemisphere to the other, but we would always see the ice-covered nighttime side in darkness. Still, exoplanet-hunting scientists have been able to tease out an amazing amount of information from the data available before- perhaps we’ll soon know if such planetary oases exist far inside the “snowline” orbiting around red dwarf stars.

Read the paper on Water-Trapped Worlds at the following link.

June 28, 2013April 26, 2016 by Elizabeth Howell

What Happens When Stars Collide? In This Case, A Newly Observed Kind of Pulsating Star

Artist's impression of the eclipsing, pulsating binary star J0247-25. (Credit: Keele University)

It sure would be interesting to watch two stars run into each other — from a safe distance, of course. One can imagine there would be quite the titanic battle going on between their competing gravitational forces, throwing off gas and matter as they collide.

They also leave behind interesting echoes, at least according to new research. A European team looked at the leftovers of one collision and found a type of pulsating star that has never been seen before.

It’s common for stars to form in groups or to be paired up, since they form from immense gas clouds. Sometimes, a red giant star in a binary system gets so big that it will bump into a companion star orbiting nearby. This crash could shave 90% of the red giant star’s mass off, but astronomers are still trying to get their heads around what happens.

Eclipsing Binaries — Artists impression of a binary star system (courtesy NASA)

“Only a few stars that have recently emerged from a stellar collision are known, so it has been difficult to study the connection between stellar collisions and the various exotic stellar systems they produce,” Keele University, which led the research, stated.

Researchers who made the find were actually on the hunt for alien planets. They turned up what is called an “eclipsing” binary system, meaning that one of the stars passes in front of the other from the perspective of Earth.

The scientists then used a high-speed camera on the Very Large Telescope in Chile called ULTRACAM. The camera is capable of taking up to 500 pictures a second to track fast-moving astronomical events.

Observations revealed that “the remnant of the stripped red giant is a new type of pulsating star,” Keele stated.

The Very Large Telescope (VLT) at ESO's Cerro Paranal observing site in the Atacama Desert of Chile, consisting of four Unit Telescopes with main mirrors 8.2-m in diameter and four movable 1.8-m diameter Auxiliary Telescopes. The telescopes can work together, in groups of two or three, to form a giant interferometer, allowing astronomers to see details up to 25 times finer than with the individual telescopes. Credit: Iztok Boncina/ESO — The Very Large Telescope (VLT) at ESO’s Cerro Paranal observing site in the Atacama Desert of Chile, consisting of four Unit Telescopes with main mirrors 8.2-m in diameter and four movable 1.8-m diameter Auxiliary Telescopes. The telescopes can work together, in groups of two or three, to form a giant interferometer, allowing astronomers to see details up to 25 times finer than with the individual telescopes. Credit: Iztok Boncina/ESO

“We have been able to find out a lot about these stars, such as how much they weigh, because they are in a binary system,” stated Pierre Maxted, an astrophysicist at Keele.

“This will really help us to interpret the pulsation signal and so figure out how these stars survived the collision and what will become of them over the next few billion years.”

The next step for the researchers will be to calculate when the star will begin cooling down and become a white dwarf, which is what is left behind after a star runs out of fuel to burn.

Check out more details of the find in Nature.

Source: Keele University

June 14, 2013October 18, 2016 by Fraser Cain

Closest Star to the Sun

This is a classic trick question. Ask a friend, “what is the closest star?” and then watch as they try to recall some nearby stars. Sirius maybe? Alpha something or other? Betelgeuse?

The answer, obviously, is the Sun; that massive ball of plasma located a mere 150 million km from Earth.

Let’s be more precise; what’s the closest star to the Sun?

Closest Star

You might have heard that it’s Alpha Centauri, the third brightest star in the sky, just 4.37 light-years from Earth.

But Alpha Centauri isn’t one star, it’s a system of three stars. First, there’s a binary pair, orbiting a common center of gravity every 80 years. Alpha Centauri A is just a little more massive and brighter than the Sun, and Alpha Centauri B is slightly less massive than the Sun. Then there’s a third member of this system, the faint red dwarf star, Proxima Centauri.

It’s the closest star to our Sun, located just a short 4.24 light-years away.

Closest Star, Proxima Centauri — Proxima Centauri

Alpha Centauri is located in the Centaurus constellation, which is only visible in the Southern Hemisphere. Unfortunately, even if you can see the system, you can’t see Proxima Centauri. It’s so dim, you need a need a reasonably powerful telescope to resolve it.

Let’s get sense of scale for just how far away Proxima Centauri really is. Think about the distance from the Earth to Pluto. NASA’s New Horizons spacecraft travels at nearly 60,000 km/h, the fastest a spacecraft has ever traveled in the Solar System. It will have taken more than nine years to make this journey when it arrives in 2015. Travelling at this speed, to get to Proxima Centauri, it would take New Horizons 78,000 years.

Proxima Centauri has been the nearest star for about 32,000 years, and it will hold this record for another 33,000 years. It will make its closest approach to the Sun in about 26,700 years, getting to within 3.11 light-years of Earth. After 33,000 years from now, the nearest star will be Ross 248.

What About the Northern Hemisphere?

Bernard's Star, one of the closest stars to the Sun — Bernard’s Star

For those of us in the Northern Hemisphere, the closest visible star is Barnard’s Star, another red dwarf in the constellation Ophiuchus. Unfortunately, just like Proxima Centauri, it’s too dim to see with the unaided eye.

The closest star that you can see with the naked eye in the Northern Hemisphere is Sirius, the Dog Star. Sirius, has twice the mass and is almost twice the size of the Sun, and it’s the brightest star in the sky. Located 8.6 light-years away in the constellation Canis Major – it’s very familiar as the bright star chasing Orion across the night sky in Winter.

How do Astronomers Measure the Distance to Stars?

They use a technique called parallax. Do a little experiment here. Hold one of your arms out at length and put your thumb up so that it’s beside some distant reference object. Now take turns opening and closing each eye. Notice how your thumb seems to jump back and forth as you switch eyes? That’s the parallax method.

To measure the distance to stars, you measure the angle to a star when the Earth is one side of its orbit; say in the summer. Then you wait 6 month, until the Earth has moved to the opposite side of its orbit, and then measure the angle to the star compared to some distant reference object. If the star is close, the angle will be measurable, and the distance can be calculated.

You can only really measure the distance to the nearest stars this way, since it only works to about 100 light-years.

The 20 Closest Stars

Here is a list of the 20 closest star systems and their distance in light-years. Some of these have multiple stars, but they’re part of the same system.

Alpha Centauri – 4.2
Barnard’s Star – 5.9
Wolf 359 – 7.8
Lalande 21185 – 8.3
Sirius – 8.6
Luyten 726-8 – 8.7
Ross 154 – 9.7
Ross 248 – 10.3
Epsilon Eridani – 10.5
Lacaille 9352 – 10.7
Ross 128 – 10.9
EZ Aquarii – 11.3
Procyon – 11.4
61 Cygni – 11.4
Struve 2398 – 11.5
Groombridge 34 – 11.6
Epison Indi – 11.8
Dx Carncri – 11.8
Tau Ceti – 11.9
GJ 106 – 11.9

According to NASA data, there are 45 stars within 17 light years of the Sun. There are thought to be as many as 200 billion stars in our galaxy. Some are so faint that they are nearly impossible to detect. Maybe, with technological improvements, scientists will find even closer stars.

June 11, 2013December 23, 2015 by Shannon Hall

A Rare Opportunity to Watch a Blue Straggler Forming

The globular cluster NGC 6388. Blue stragglers may clearly be seen around the edges. More are hidden within the central core. Credit: ESO

A unique and enigmatic variety of stars known as blue stragglers appear to defy the normal stellar aging process. Discovered in globular clusters, they appear much younger than the rest of the stellar population. Since their discovery in 1953, astronomers have been asking the question: how do these stars regain their youth?

For years, two theories have persisted. The first theory suggests that two stars collide, forming a single more massive star. The second theory proposes that blue stragglers emerge from binary pairs. As the more massive star evolves and expands, it blows material onto the smaller star. In both theories, the star grows steadily more massive and bluer – it regains its youth.

But now, a surprising finding may lend credence to the second theory. Astronomers at the Nicolaus Copernicus Astronomical Center in Poland recently observed a blue straggler caught in the midst of forming!

The binary system that was studied, known as M55-V60, is located within the globular cluster M55. Dr. Michal Rozyczka, one of the research scientists on the project, told Universe Today, “The system is a showcase example of a blue straggler formed via the theoretically predicted peaceful mass exchange between its components.”

The team used both photometric (the overall light from the system) and spectroscopic (the light spread out into a range of wavelengths) observations. The photometric data revealed the light curve – the change in brightness due to one star passing in front of the other – of the system. This provided evidence that the astronomers were looking at a binary system.

From the spectroscopic data, shifts in wavelength reveal the velocity (along the line of sight) of a source. The research team noted that the system’s center of mass was moving with respect to the binary system. This will occur in a semi-detached binary system, where mass transfers from one star to the other. As it does this, the center of mass will follow the mass-transfer.

From both photometric and spectroscopic observations (which covered more than 10 years!) the team was able to verify that this object is not only a binary, but a semi-detached binary, residing at the edge of M55.

An artist's conception of how a blue straggler may form from a binary system. Credit:NASA/ESA — An artist’s conception of how a blue straggler may form from a binary system. Credit: NASA/ESA

“The system is semi-detached with the less massive (secondary) component filling its Roche lobe,” explained Dr. Rozyczka. “The secondary has a tearlike shape, with the tip of the tear directed toward the more massive primary. A stream of gas flows out of the tip along a curved path and hits the primary.”

How do we know that it is in fact a blue straggler? The simple answer is that the secondary star, with is gaining mass, appears bluer than normal. This blue straggler is clearly in the process of forming. It is the second observation of such a formation, with the first being V228 in the globular cluster: 47 Tuc.

This research verifies that semi-detached binaries are a viable formation mechanism for blue stragglers. The binary was discovered by happenstance, in a project aimed at determining accurate ages and distances of nearby clusters. It’s certainly a surprising result from the survey.

The results will be published in Acta Astronomica, a peer-reviewed scientific journal located in Poland (preprint available here).

June 3, 2013October 16, 2016 by Fraser Cain

How Many Stars are There in the Universe?

When we look at the night sky, filled with stars, it’s hard to resist counting. Just with the unaided eye, in dark skies, you can see a few thousand.

How many stars are there in the entire Universe? Before we get to that massive number, let’s consider what you can count with the tools available to you.

Perfect vision in dark skies allows us to see stars down to about magnitude 6. But to really make an accurate census of the total number of stars, you’d need to travel to both the Northern and Southern Hemispheres, since only part of the sky is visible from each portion of the Earth. Furthermore, you’d need to make your count over several months, since a portion of the sky is obscured by the Sun. If you had perfect eyesight and traveled to completely dark skies in both the Northern and Southern Hemispheres, and there was no Moon, you might be able to get to count up almost 9,000 stars.

With a good pair of binoculars, that number jumps to about 200,000, since you can observe stars down to magnitude 9. A small telescope, capable of resolving magnitude 13 stars will let you count up to 15 million stars. Large observatories could resolve billions of stars.

But how many stars are out there? How many stars are there in the Milky Way?

According to astronomers, our Milky Way is an average-sized barred spiral galaxy measuring up to 120,000 light-years across. Our Sun is located about 27,000 light-years from the galactic core in the Orion arm. Astronomers estimate that the Milky Way contains up to 400 billion stars of various sizes and brightness.

A few are supergiants, like Betelgeuse or Rigel. Many more are average-sized stars like our Sun. The vast majority of stars in the Milky Way are red dwarf stars; dim, low mass, with a fraction of the brightness of our Sun.

As we peer through our telescopes, we can see fuzzy patches in the sky which astronomers now know are other galaxies like our Milky Way. These massive structures can contain more or less stars than our own Milky Way.

There are spiral galaxies out there with more than a trillion stars, and giant elliptical galaxies with 100 trillion stars.
And there are tiny dwarf galaxies with a fraction of our number of stars.

So how many galaxies are there?

According to astronomers, there are probably more than 170 billion galaxies in the observable Universe, stretching out into a region of space 13.8 billion light-years away from us in all directions.

And so, if you multiply the number of stars in our galaxy by the number of galaxies in the Universe, you get approximately 10²⁴ stars. That’s a 1 followed by twenty-four zeros.

That’s a septillion stars.

But there could be more than that.

It’s been calculated that the observable Universe is a bubble of space 47 billion years in all directions.

It defines the amount of the Universe that we can see, because that’s how long light has taken to reach us since the Big Bang.

This is a minimum value, the Universe could be much bigger – it’s just that we can’t ever detect those stars because they’re outside the observable Universe. It’s even possible that the Universe is infinite, stretching on forever, with an infinite amount of stars. So add a couple more zeros. Maybe an infinite number of zeroes.

That’s a lot of stars in the Universe.

Additional Resources:
How Many Stars Can you See?
Astronomy Cast: How Big is the Universe?
How Big is Our Observable Universe
Astronomy Cast: The Observable Universe
How Many Galaxies in the Universe?

May 29, 2013April 26, 2016 by Elizabeth Howell

Why Are Dying Stars in 47 Tucanae Cooling Off So Slowly?

The Hubble Space Telescope is going to be used to settle an argument. It’s a conflict between computer models and what astronomers are seeing in a group of stars in 47 Tucanae.

White dwarfs — the dying embers of stars who have burnt off all their fuel — are cooling off slower than expected in this southern globular cluster, according to previous observations with the telescope’s Wide Field Camera and Advanced Camera for Surveys.

Puzzled astronomers are now going to widen that search in 47 Tucanae (which initially focused on a few hundred objects) to 5,000 white dwarfs. They do have some theories as to what might be happening, though.

White dwarfs, stated lead astronomer Ryan Goldsbury from the University of British Columbia, have several factors that chip in to the cooling rate:

The Hubble Space Telescope. Image credit: NASA, tweaked by D. Majaess.

– High-energy particle production from the white dwarfs;

– What their cores are made up of;

– What their atmospheres are made up of;

– Processes that bring energy from the core to the surface.

Somewhere, somehow, perhaps one of those factors is off.

This kind of thing is common in science, as astronomers create these programs according to the best educated guesses they can make with respect to the data in front of them. When the two sides don’t jive, they do more observations to refine the model.

“The cause of this difference is not yet understood, but it is clear that there is a discrepancy between the data and the models,” stated the Canadian Astronomical Society (CASCA) and the University of British Columbia in a press release.

Since the white dwarfs are in a cluster that presumably formed from the same cloud of gas, it allows astronomers to look at a group of stars at a similar distance and to determine the distribution of masses of stars within the cluster.

“Because all of the white dwarfs in their study come from a single well-studied star cluster, both of these bits of information can be independently determined,” the release added.

You can read the entire article on the previous Hubble research on 47 Tucanae at the Astrophysical Journal.

Today’s announcement took place during the annual meeting of CASCA, which is held this year in Vancouver.

Source: CASCA/UBC

May 29, 2013December 23, 2015 by Elizabeth Howell

Suddenly Slowing Star Could Give Hints Of Its Interior

Artist's conception of a neutron star flare. Credit: University of California Santa Cruz

Why would a spinning star suddenly slow down? Even after writing a scientific paper about the phenomenon, astronomers still appear to be in shock-and-awe mode about what they saw.

“I looked at the data and was shocked — the … star had suddenly slowed down,” stated Rob Archibald, a graduate student at McGill University in Montreal. “These stars are not supposed to behave this way.”

Archibald led a group that was observing a neutron star, a type of really, really dense object created after huge stars run out of gas and collapse. The studied star (called 1E 2259+586, if you’re curious) has a massive magnetic field that places it in a subcategory of neutron stars called magnetars.

Anyway, the astronomers were watching over the magnetar with the NASA Swift X-ray telescope, just to get a sense of the star’s rotation and also to keep an eye out for the odd X-ray explosion commonly seen in stars of this type. But to see its spin rate reduce — that was definitely something unexpected.

An artistic impression of a magnetar with a very complicated magnetic field at its interior and a simple small dipolar field outside. Credits: ESA - Author: Christophe Carreau — An artistic impression of a magnetar with a very complicated magnetic field at its interior and a simple small dipolar field outside. Credits: ESA – Author: Christophe Carreau

Previous neutron star observations have showed them suddenly rotating faster (as if spinning up to several hundred times a second wasn’t enough.) This maneuver is called a glitch, and is thought to happen because the neutron has some sort of fluid (sometimes called a “superfluid”) inside that drives the rotation.

So now, the astronomers had evidence of an “anti-glitch”, a star slowing down instead of speeding up. It wasn’t by much (just a third of a part per million in the seven-second rotation rate), but while it happened they also saw X-rays substantially increase from the magnetar. Astronomers believe that something major happened either inside, or near the surface of the star.

The magnetic field surrounding the mysterious magnetar (NASA) — The magnetic field surrounding a magnetar (NASA)

And, astronomers added, if they can figure out what is happening, it could shed some light on what exactly is going on in that dense interior. Maybe the fluid is rotating at different rates, or something else is going on.

“Such behaviour is not predicted by models of neutron star spin-down and, if of internal origin, is suggestive of differential rotation in the magnetar, supporting the need for a rethinking of glitch theory for all neutron stars,” read a paper on the results.

The work was released today (May 29) at the Canadian Astronomical Society (CASCA)’s annual meeting, held this year in Vancouver.

You can read the entire paper in Nature.

Credit: CASCA/McGill University