Elizabeth Howell is the senior writer at Universe Today. She also works for Space.com, Space Exploration Network, the NASA Lunar Science Institute, NASA Astrobiology Magazine and LiveScience, among others. Career highlights include watching three shuttle launches, and going on a two-week simulated Mars expedition in rural Utah. You can follow her on Twitter @howellspace or contact her at her website.
False color radar topographical map of Venus provided by Magellan. Credit: Magellan Team/JPL/NASA
Venus was once considered a twin to Earth, as it’s roughly the same size and is relatively close to our planet. But once astronomers looked at it seriously in the past half-century or so, a lot of contrasts emerged. The biggest one — Venus is actually a hothouse planet with a runaway greenhouse effect, making it inhospitable to life as we know it. Here are some more interesting facts about Venus.
1. Venus’ atmosphere killed spacecraft dead very quickly: You sure don’t want to hang around on Venus’ surface. The pressure there is so great that spacecraft need shielding to survive. The atmosphere is made up of carbon dioxide with bits of sulfuric acid, NASA says, which is deadly to humans. And if that’s not bad enough, the temperature at the surface is higher than 470 degrees Celsius (880 degrees Fahrenheit). The Soviet Venera probes that ventured to the surface decades ago didn’t last more than two hours.
2. But conditions are more temperate higher in the atmosphere: While you still couldn’t breathe the atmosphere high above Venus’ surface, at about 50 kilometers (31 miles) you’ll at least find the same pressure and atmosphere density as that of Earth. A very preliminary NASA study suggests that at some point, we could deploy airships for humans to explore Venus. And the backers suggest it may be more efficient to go to Venus than to Mars, with one large reason being that Venus is closer to Earth.
Artist’s conception of the High Altitude Venus Operational Concept (HAVOC) mission, a far-out concept being developed by NASA, approaching the planet. Credit: NASA Langley Research Center/YouTube (screenshot)
3. Venus is so bright it is sometimes mistaken for a UFO: The planet is completely socked in by cloud, which makes it extremely reflective to observers looking at the sky on Earth. Its brightness is between -3.8 and -4.8 magnitude, which makes it brighter than the stars in the sky. In fact, it’s so bright that you can see it go through phases in a telescope — and it can cast shadows! So that remarkable appearance can confuse people not familiar with Venus in the sky, leading to reports of airplanes or UFOs.
4. And those clouds mean you can’t see the surface: If you were to look at Venus with your eyes, you wouldn’t be able to see its surface. That’s because the clouds are so thick that they obscure what is below. NASA got around that problem when it sent the Magellan probe to Venus for exploration in the 1990s. The probe orbited the planet and got a complete surface picture using radar.
Artist’s impression of the surface of Venus Credit: ESA/AOES
5. Venus has volcanoes and a fresh face: Venus has fresh lava flows on its surface, which implies that volcanoes erupted anywhere from the past few hundred years to the past three million years. What this means is there are few impact craters on the surface, likely because the lava flowed over them and filled them in. While scientists believe the volcanoes are responsible, the larger question is how frequently this occurs.
6. Venus has a bizarre rotation: Venus not only rotates backwards compared to the other planets, but it rotates very slowly. In fact, a day on Venus (243 days) lasts longer than it takes the planet to orbit around the Sun (225 days). Even more strangely, the rotation appears to be slowing down; Venus is turning 6.5 minutes more slowly in 2014 than in the early 1990s. One theory for the change could be the planet’s weather; its thick atmosphere may grind against the surface and slow down the rotation.
Artist’s conception of Venus Express doing an aerobraking maneuver in the atmosphere in 2014. Credit: ESA–C. Carreau
7. Venus has no moons or rings: The two planets closest to the Sun have no rings or moons, which puts Venus in the company of only one other world: Mercury. Every other planet in the Solar System has one or the other, or in many cases both! Why this is is a mystery to scientists, but they are doing as much comparison of different planets as possible to understand what’s going on.
8. Venus appears to be a spot where spacecraft go to extremes: We briefly mentioned the Venera probes that landed on the surface, but that’s not the only unusual spacecraft activity at Venus. In 2014, the European Space Agency put an orbiter — that’s right, a spacecraft not designed to survive the atmosphere — into the upper parts of Venus’ dense atmosphere. Venus Express did indeed survive the encounter (before it ran out of gas), with the goal of providing more information about how the atmosphere looks at high altitudes. This could help with landings in the future.
As you can see, Venus is an interesting, mysterious, and extremely hostile world. With such a corrosive atmosphere, such incredible heat, a volcanically-scarred surface, and thick clouds of toxic gas, one would have to be crazy to want to live there. And yet, there are some who believe Venus could be terraformed for human use, or at the very least explored using airships, in the coming generations.
But that’s the thing about interesting places. Initially, they draw their fair share of research and attention. But eventually, the dreamers and adventurers come.
Caloris in Color – An enhanced-color view of Mercury from the cameras on board the MESSENGER spacecraft. The circular, orange area near the center-top of the disc is Caloris Basin. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington
Close by the Sun is Mercury, a practically atmosphere-like world that has a lot of craters. Until NASA’s MESSENGER spacecraft arrived there in 2008, we knew very little about the planet — only part of it had been imaged! But now that the spacecraft has been circling the planet for a few years, we know a heck of a lot more. Here is some stuff about Mercury that’s useful to know.
1. Mercury has water ice and organics.
This may sound surprising given that the planet is so close to the Sun, but the ice is in permanently shadowed craters that don’t receive any sunlight. Organics, a building block for life, were also found on the planet’s surface. While Mercury doesn’t have enough atmosphere and is too hot for life as we know it, finding organics there demonstrates how those compounds were distributed throughout the solar system. There’s also quite a bit of sulfur on the surface, something that scientists are still trying to understand since no other planet in the Solar System has it in such high concentrations.
2. The water ice appears younger than we would expect.
Close examination of the ice shows sharp boundaries, which implies that it wasn’t deposited that long ago; if it was, the ice would be somewhat eroded and mixed in with Mercury’s regolith surface. So somehow, the ice perhaps came there recently — but how? What’s more, it appears the ice deposits on the Moon and the ice deposits on Mercury are different ages, which could imply different conditions for both of the bodies.
A forced perspective view of Mercury’s north pole (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)
3. Mercury has an atmosphere that changes with its distance to the Sun.
The planet has a very thin atmosphere that is known as an “exosphere” (something that is also present on the Moon, for example.) Scientists have detected calcium, sodium and magnesium in it — all elements that appear to change in concentration as the planet gets closer and further from the Sun in its orbit. The changes appear to be linked to how much solar radiation pressure falls on the planet.
4. Mercury’s magnetic field is different at its poles.
Mercury is somehow generating a magnetic field in its interior, but it’s quite weak (just 1% that of Earth’s). That said, scientists have observed differences in the north and the south pole magnetic strength. Specifically, at the south pole, the magnetic field lines have a bigger “hole” for charged particles from the Sun to strike the planet. Those charged particles are believed to erode Mercury’s surface and also to contribute to its composition.
Illustration of MESSENGER in orbit around Mercury (NASA/JPL/APL)
5. Despite Mercury’s weak magnetic field, it behaves similarly to Earth’s.
Specifically, the magnetic field does deflect charged particles similarly to how Earth does, creating a “hot flow anomaly” that has been observed on other planets. Because particles flowing from the Sun don’t come uniformly, they can get turbulent when they encounter a planet’s magnetic field. When plasma from the turbulence gets trapped, the superheated gas also generates magnetic fields and creates the HFA.
6. Mercury’s eccentric orbit helped prove Einstein’s theory of relativity.
Mercury’s eccentric orbit relative to the other planets, and its close distance to the Sun, helped scientists confirm Einstein’s general theory of relativity. Simply put, the theory deals with how the light of a star changes when another planet or star orbits nearby. According to Encyclopedia Britannica, scientists confirmed the theory in part by reflecting radar signals off of Mercury. The theory says that the path of the signals will change slightly if the Sun was there, compared to if it was not. The path matched what general relativity predicted.
A hot flow anomaly, or HFA, has been identified around Mercury (Credit: NASA/Duberstein)
7. Mercury is hard to spot in the sky, but has been known for millennia.
Mercury tends to play peekaboo with the Sun, which makes it somewhat of an observing challenge. The planet rises or sets very close to when the Sun does, which means amateur astronomers are often fighting against twilight to observe the tiny planet. That being said, the ancients had darker skies than we did (no light pollution) and were able to see Mercury pretty well. So the planet has been known for thousands of years, and was linked to some of the gods in ancient cultures.
8. Mercury has no moons or rings.
Scientists are still trying to understand how the Solar System formed, and one of the ways they do so is by comparing the planets. Interesting to note about Mercury: it has no rings or moons, which makes it different from just about every other planet in our Solar System. The exception is Venus, which also has no moons or rings.
A full Moon flyby, as seen from Paris, France. Credit and copyright: Sebastien Lebrigand.
Shining like a beacon in Earth’s sky is the Moon. We’ve seen so much of it in our lifetimes that it’s easy to take it for granted; even the human landings on the Moon in the 1960s and 1970s were eventually taken for granted by the public.
Fortunately for science, we haven’t stopped looking at the Moon in the decades after Neil Armstrong took his first step. Here are a few things to consider about Earth’s closest big neighbor.
Artist's impression of New Horizons' encounter with Pluto and Charon. Credit: NASA/Thierry Lombry
As the New Horizons spacecraft gets closer and closer to Pluto in advance of its July 2015 flyby, it manages to and gathers more and more information. As a result, we learning more about the dwarf planet on an almost regular basis.
Pluto is now becoming more to the public than just the planet that no longer was; before long, we’ll be able to understand much about its atmosphere, its moons and how it fits into the story of the Solar System’s history. Here are some of the most interesting things we know about Pluto so far.
1. Its definition of “dwarf planet” is controversial: Back in 2006, the International Astronomical Union deemed Pluto is a dwarf planet and not a planet. The reasoning came after a few other objects were discovered far out in the Solar System that are close to Pluto’s size. That said, the principal investigator for New Horizons, Alan Stern, does not agree with the definition. At the time of the vote, he pointed out that the IAC’s definition of planet was not completely true of any larger body; for example, Earth does not clear the entire neighborhood of debris, which is one of the parts of the definition.
2. Pluto has several moons: For decades, astronomers knew of Pluto and its moon, Charon. The two are so close in size that some people considered the system a double planet, but now that’s thrown in doubt with the dwarf planet designation. In any case, in the last decade humanity has discovered several more moons as telescope resolution and observing techniques improved. The other moons are called Nix, Hydra, Kerberos and Styx. For now we don’t know much about these smaller moons because it’s so difficult to resolve features on their tiny size.
HST Image of Pluto-Charon system. Also shown are Nix and Hydra. Image Credit: NASA/ESA
3. Charon might have an ocean on it: It seems unbelieveable that Charon could have an ocean given it’s so far away from the Sun, but at least one study suggests that it could be possible. Essentially, the tidal force imparted by Pluto’s gravity early in Charon’s history could have stretched the moon’s insides and warmed them up enough to create liquid. That said, it’s also possible that the ocean is now frozen as Charon’s orbit is not as eccentric as it was in the past.
4. Charon’s formation could have spawned the other moons: As with our own Moon, some scientists believe Charon was created after a large object smashed into Pluto billions of years ago. This would have created a chain of debris circling the dwarf planet, which eventually coalesced into Charon. However, the other moons we know of near Pluto have almost exact resonances with Charon. This suggests that they also formed from the debris, one study says.
Footage of Charon, captured by NASA’s New Horizons spacecraft, taken in July 2014. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
5. Pluto has an atmosphere: Pluto is a tiny world, but like the Moon and Mercury it does have a very tenuous atmosphere that is called an “exosphere.” Astronomers first spotted signs of it in 1985. As Pluto passed in front of a star, they saw the star very slightly dim before Pluto completely blocked the star. The composition of this atmosphere is mostly made up of nitrogen and methane, and it freezes when Pluto is furthest from the Sun.
6. Pluto can get closer to the Sun than Neptune: We used to think of Pluto as the furthest planet from the Sun, but in reality its orbit is so eccentric that it comes closer to the Sun than Neptune. According to NASA, its average distance from the Sun is 39.5 astronomical units (Earth-Sun distances), but it can come as close as 29.7 AU and as far away as 49.7 AU. It was last “inside” Neptune’s orbit between 1979 and 1999.
Pluto’s surface as viewed from the Hubble Space Telescope in several pictures taken in 2002 and 2003. Though the telescope is a powerful tool, the dwarf planet is so small that it is difficult to resolve its surface. Astronomers noted a bright spot (180 degrees) with an unusual abundance of carbon monoxide frost. Credit: NASA
7. Astronomers think Pluto looks a lot like Neptune’s moon, Triton: Let’s be clear that Triton and Pluto have very different histories; for example, Triton was likely captured by Neptune long ago, an event that drastically altered its surface and its insides. But Pluto and Triton likely do have some similarities: the frozen volatiles (elements with low boiling points), the faint nitrogen atmospheres, and their similar composition of ice and rock. Scientists are pulling out old Voyager 2 pictures to make the comparisons as Pluto pictures arrive from New Horizons.
8. Pluto could have a ring system: It’s not a guarantee, but at least one research team suggests that debris floating around Pluto could coalesce into a faint ring system. This wouldn’t be a large surprise, by the by, as we already know of at least one asteroid that has rings — so it is possible. Researchers on New Horizons will also be on the lookout for more moons and interesting features on Pluto’s surface such as cracks.
Artist's conception of the solar system, often used in the Eyes on the Solar System 3D Simulator. Credit: NASA
While most of us are stuck on planet Earth, we’re lucky enough to have a fairly transparent atmosphere. This allows us to look up at the sky and observe changes. The ancients noticed planets wandering across the sky, and occasional visitors such as comets.
Thousands of years ago, most thought the stars ruled our destiny. Today, however, we can see science at work in the planets, asteroids and comets close to home. So why take a look at the Solar System? What can it teach us?
1. The definition of a planet and a moon is fuzzy.
We all know of that famous International Astronomical Union vote in 2006 where Pluto was demoted from planethood into a newly created class called “dwarf planets.” But the definition drew controversy among some, who pointed out that no planet — dwarf or otherwise — perfectly clears the neighborhood in its orbit of asteroids, for example. Moons are considered to orbit around planets, but that doesn’t cover situations such as moons orbiting asteroids or double planets, for example. Goes to show you the Solar System requires more study to figure this out.
2. Comets and asteroids are leftovers.
No, we don’t mean leftovers to eat — we mean leftovers of what the Solar System used to look like. So while it’s easy to get distracted by the weather and craters and prospects for life on planets and moons, it’s important to remember that we must also pay attention to the smaller bodies. Comets and asteroids, for example, could have brought organics and water ice to our own planet — providing what we need for life.
Four images of Comet 67P/Churyumov–Gerasimenko taken on Nov. 30, 2014 by the orbiting Rosetta spacecraft. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
3. The planets are all on the same “plane” and orbit in the same direction.
When considering the IAU’s definition of planets, we come up with eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. You’ll notice that these bodies tend to follow the same path in the sky (called the ecliptic) and that they orbit the Sun in the same direction. That supports the leading theory for the Solar System’s formation, which is that the planets and moons and Sun formed from a large gas and dust cloud that condensed and spun.
4. We’re nowhere near the center of the galaxy.
We can measure vast distances across the universe by looking at things such as “standard candles” — a type of exploding stars that tend to have the same luminosity, which makes it easier to predict how far away they are from us. At any rate, looking at our neighborhood, we’ve been able to figure out we’re nowhere near the Milky Way galaxy’s center. We’re about 165 quadrillion miles away from the center supermassive black hole, NASA says, which is probably a good thing.
A still photo from an animated flythrough of the universe using SDSS data. This image shows our Milky Way Galaxy. The galaxy shape is an artist’s conception, and each of the small white dots is one of the hundreds of thousands of stars as seen by the SDSS. Image credit: Dana Berry / SkyWorks Digital, Inc. and Jonathan Bird (Vanderbilt University)
5. But the Solar System is bigger than you think.
Beyond the orbit of Neptune (the furthermost planet), it takes a long time to leave the Solar System. In 2012, some 35 years after leaving Earth on a one-way trip to the outer solar Solar System, Voyager 1 passed through the area where the Sun’s magnetic and gas environment gives way to that of the stars, meaning that it is interstellar space. That was an astounding 11 billion miles (17 billion kilometers) away from Earth, or roughly 118 equivalent Earth-sun distances (astronomical units).
6. The Sun is hugely massive.
Just how massive? 99.86% of the Solar System’s mass is in our local star, which goes to show you where the real heavyweight is. The Sun is made up of hydrogen and helium, which shows you that these gases are far more abundant in our neighborhood (and the Universe generally) than the rocks and metals we are more familiar with here on Earth.
Solar prominences and filaments on the Sun on September 18, 2014, as seen with a hydrogen alpha filter. Credit and copyright: John Chumack/Galactic Images.
7. We haven’t finished searching for life here.
So we know for sure that life exists on Earth, but that doesn’t rule out a whole bunch of other places. Mars had water flowing on it in the ancient past, and has frozen water at its poles — making astrobiologists think it might be a good candidate. There also are a range of icy moons that could have oceans with life below the surfaces, such as Europa (at Jupiter) and Enceladus (at Saturn). There’s also the interesting world of Titan, which has “prebiotic chemistry” — chemistry that was a precursor to life — on its surface.
8. We can use the Solar System to better understand exoplanets.
Exoplanets are so far away, and so small in our telescopes, that it’s difficult to see very much detail in their atmospheres. But by looking at the chemistry of Jupiter, for example, we can make some predictions about gas giants further afield. If we look at Earth and Neptune, we can get a better sense of the range of planetary sizes on which life could exist (those “super-Earths” and “mini-Neptunes” you sometimes hear mentioned.) And even looking at where water freezes in our own Solar System can help us better understand the ice line in other locations.
We have written articles about the solar system for Universe Today. Here are facts about the planets in the Solar System. We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.
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.
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?
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.
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.
