Stars are born in molecular clouds, massive clouds of hydrogen that can contain millions of stellar masses of material. But how do molecular clouds form? There are different theories and models of that process, but the cloud formation is difficult to observe.
A new study is making some headway, and showing how the process occurs more rapidly than thought.
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).
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.
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).
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.
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.”
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.
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.
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.
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.”
Greetings, fellow SkyWatchers! It’s going to be a great week for lunar studies and an even better time to study some interesting single stars. Need more? Then keep an eye on the skies as the Delta Leonid meteor shower heats up towards its later week peak. Get out those binoculars and telescopes and I’ll see you in the backyard…
Monday, February 27 – With tonight’s Moon in a much higher position to observe, let’s begin with an investigation of Mare Fecunditatis – the Sea of Fertility. Stretching 1463 kilometers in diameter, the combined area of this mare is equal in size to the Great Sandy Desert in Australia – and almost as vacant in interior features. It is home to glasses, pyroxenes, feldspars, oxides, olivines, troilite and metals in its lunar soil, which is called regolith. Studies show the basaltic flow inside of the Fecunditatis basin perhaps occurred all at once, making its chemical composition different from other maria. The lower titanium content means it is between 3.1 and 3.6 billion years old!
The western edge of Fecunditatis is home to features we share terrestrially – grabens. These down-dropped areas of landscape between parallel fault lines occur where the crust is stretched to the breaking point. On Earth, these happen along tectonic plates, but on the Moon they are found around basins. The forces created by lava flow increase the weight inside the basin, causing a tension along the border which eventually fault and cause these areas. Look closely along the western shore of Fecunditatis where you will see many such features.
Today is the birthday of Bernard Lyot. Born in 1897, Lyot went on to become the inventor of the coronagraph in 1930. By all accounts, Lyot was a wonderful and generous man who sadly died of a heart attack when returning from a trip to view a total eclipse. Although we cannot hand you a corona, we can show you a star that wears its own gaseous envelope.
Let’s go to our maps west of M36 and M38 to identify AE Aurigae. As an unusual variable, AE is normally around 6th magnitude and resides approximately 1600 light years distant. The beauty in this region is not particularly the star itself but the faint nebula in which it resides known as IC 405, an area of mostly dust and very little gas. What makes this view so entertaining is that we are looking at a “runaway” star. It is believed that AE once originated from the M42 region in Orion. Cruising along at a very respectable speed of 80 miles per second, AE flew the “stellar nest” some 2.7 million years ago! Although IC 405 is not directly related to AE, there is evidence within the nebula that areas have been cleared of their dust by the rapid northward motion of the star. AE’s hot, blue illumination and high energy photons fuel what little gas is contained within the region. Its light also reflects off the surrounding dust. Although we cannot “see” with our eyes like a photograph, together the pair forms an outstanding view for the small backyard telescope and it is known as “The Flaming Star.”
Tuesday, February 28 – Since the stars of our study constellation of Monoceros are quite dim when the Moon begins to interfere, why not spend a few days really taking a look at the Moon’s surface and familiarizing yourself with its many features? Tonight would be a great time for us to explore “The Sea of Nectar.” At around 1000 meters deep, Mare Nectaris covers an area of the Moon equal to that of the Great Sandhills in Saskatchewan, Canada. Like all maria, it is part of a gigantic basin that is filled with lava, and evidence of grabens exists along its western basin edge. While Nectaris’ basaltic flows appear darker than those in most maria, it is one of the older formations on the Moon and as the terminator progress, you’ll be able to see where ejecta belonging to Tycho crosses its surface. For now? Let’s have a closer look at the mare itself and its surrounding craters… Enjoy these many features which are also lunar challenges – and we’ll be back to study each later in the year!
Now, let’s have a look about a fistwidth north-northwest of Sirius – for Beta Monocerotis. Discovered by Sir William Herschel in 1781, Beta is perhaps one of the most outstanding triple systems in the sky, with each of its three bright, white components near equal magnitude. Residing about 100-200 light-years away, these identical spectral type stars are separated by no more than 400 AU and don’t appear to have changed positions since measured by Struve in 1831. Although you won’t be able to split this system with binoculars, even a small telescope will pick apart their brilliancy and make Beta a star to remember!
Wednesday, February 29 – Tonight let your imagination sweep you away as we go mountain climbing – on the Moon! Tonight all of Mare Serenitatis will be revealed and along its northwestern shore lie some of the most beautiful mountain ranges you’ll ever view – The Caucasus to the north and the Apennines to the south. Like its earthly counterpart, the Caucasus Mountain range stretches almost 550 kilometers and some of its peaks reach upwards to 6 kilometers – a summit as high as Mount Elbrus!
Slightly smaller than its terrestrial namesake, the lunar Apennine mountain range extends some 600 kilometers with peaks rising as high as 5 kilometers. Be sure to look for Mons Hadley, one of the tallest peaks that you will see at the northern end of this chain. It rises above the surface to a height of 4.6 kilometers, making that single mountain about the size of asteroid Toutatis.
Thursday, March 1 – In 1966 Venera 3 became the first craft to touch another world as it impacted Venus. Although its communications failed before it could transmit data, it was a milestone achievement.
George Abell was born on this day in 1927. Abell was the man responsible for cataloging 2712 clusters of galaxies from the Palomar sky survey, which was completed in 1958. Using these plates, Abell put forth the idea that the grouping of such clusters distinguished the arrangement of matter in the universe. He developed the “luminosity function,” which shows relationship between brightness and number of members in each cluster, allowing you to infer their distances. Abell also discovered a number of planetary nebulae and developed the theory (along with Peter Goldreich) of their evolution from red giants. Abell was a fascinating lecturer and a developer of many television series dedicated to explaining science and astronomy in a fun and easy to understand format. He was also a president and member of the Board of Directors for the Astronomical Society of the Pacific, as well as serving in the American Astronomical Society, the Cosmology Commission of the International Astronomical Union, and he accepted editorship of the Astronomical Journal just before he died.
Tonight your lunar assignments are relatively easy. We will begin by identifying “The Sea of Vapors.” Look for Mare Vaporum on the southwest shore of Mare Serenitatis. Formed from newer lava flow inside an old crater, this lunar sea is edged to its north by the mighty Apennine Mountains. On its northeastern edge, look for the now washed-out Haemus Mountains. Can you see where lava flow has reached them? This lava has come from different time periods and the slightly different colorations are easy to spot even with binoculars.
Further south and edged by the terminator is Sinus Medii – “The Bay in the Middle.” With an area about the size of both Massachusetts and Connecticut, this lunar feature is the mid-point of the visible lunar surface. In 1930, experiments were underway to test this region for surface temperature – a project begun by Lord Rosse in 1868. Surprisingly enough, results of the two studies were very close, and during full daylight temperatures in Sinus Medii can reach the boiling point as evidenced by Surveyors 4 and 6 – which landed near its center.
Now take a hop north of Mare Vaporum for a look at “The Rotten Swamp” – Palus Putredinus. More pleasingly known as the “Marsh of Decay,” this nearly level surface of lava flow is also home to a mission – the hard-landing of Lunik 2. On September 13, 1959 astronomers in Europe reported seeing the black dot of the crashing probe. The event lasted for nearly 300 seconds and spread over an area of 40 kilometers
Friday, March 2 – Tonight it’s time to relax and enjoy the Delta Leonid meteor shower. Burning through our atmosphere at speeds of up to 24 kilometers per second, these slow travelers will seem to radiate from a point around the middle of Leo’s “back.” The fall rate is rather slow at around 5 per hour, but they are still worth keeping a watch for!
Tonight let’s return again to the lunar surface to study how the terminator has moved and take a close look at the way features change as the Sun brightens the moonscape. Can you still see Langrenus? How about Theophilus, Cyrillus and Catharina? Does Posidonius still look the same? Each night features further east become brighter and harder to distinguish – yet they also change in subtle and unexpected ways. We’ll look at that in the days ahead, but tonight let’s walk the terminator as one of the most beautiful features has now come into view – “The Bay of Rainbows.” Sinus Iridum’s C-shape is easily recognizable in even small binoculars – yet there are a wonderland of small details in and around the area for the small telescope that we’ll study as the year goes by.
Saturday, March 3 – Tonight’s bright skies are brought to you by the Moon! Have you noticed how difficult it is to see any stars belonging to Monoceros with these conditions? Don’t worry. We’ll be back. For now, let’s continue onwards with our lunar studies as we locate the emerging “Sea Of Islands.” Mare Insularum will be partially revealed tonight as one of the most prominent of lunar craters – Copernicus – now comes into view. While only a small section of this reasonably young mare is now visible southeast of Copernicus, the lighting will be just right to spot its many different colored lava flows. To the northeast is a lunar club challenge: Sinus Aestuum. Latin for the Bay of Billows, this mare-like region has an approximate diameter of 290 kilometers, and its total area is about the size of the state of New Hampshire. Containing almost no features, this area is low albedo – providing very little surface reflectivity.
Tonight let’s try a lovely triple star system – Beta Monocerotis. Located about a fist width northwest of Sirius, Beta is a distinctive white star with blue companions. Separated by about 7 arc seconds, almost any magnification will distinguish Beta’s 4.7 magnitude primary from its 5.2 magnitude secondary to the southeast. Now, add a little power and you’ll see the fainter secondary has its own 6.2 magnitude companion less than 3 arc seconds away to the east.
Before you call it a night, be sure to have a quick look at Mars. Right now the red planet is at opposition and can be seen from sunset until sunrise in the constellation of Leo. You may have also noticed that it is dimming slightly, too. It has now reached an estimated -1.23 magnitude. Be sure to look for wonderful features like Sytris Major and the polar caps!
Sunday, March 4 – In 1835, Giovanni Schiaparelli opened his eyes for the very first time and opened ours with his accomplishments! As the director of the Milan Observatory, Schiaparelli (and not Percival Lowell) was the fellow who popularized the term “Martian canals” somewhere around the year 1877. Far more importantly, Schiaparelli was the man who made the connection between the orbits of meteoroid streams and the orbits of comets almost eleven years earlier!
Tonight let’s turn binoculars or telescopes toward the southern lunar surface as we set out to view one of the most unusually formed craters – Schiller. Located near the lunar limb, Schiller appears as a strange gash bordered on the southwest in white and black on the northeast. This oblong depression might be the fusion of two or three craters, yet shows no evidence of crater walls on its smooth floor. Schiller’s formation still remains a mystery. Be sure to look for a slight ridge running along the spine of the crater to the north through the telescope. Larger scopes should resolve this feature into a series of tiny dots.
Let’s try our hand at Beta Orionis … the bright, blue/white star in the southwestern corner of Orion. As you may have noticed for the most part – the brighter the stars are, the closer they are. Not so Rigel! As the seventh brightest star in the sky, it breaks all the “rules” by being an amazing 900 light years away! Can you imagine what an awesome supergiant this white hot star really is? Rigel is actually one of the most luminous stars in our galaxy and if it were as close as Sirius it would be 20% as bright as tonight’s Moon! As an added bonus, most average backyard telescopes can also reveal Rigel’s 6.7 magnitude blue companion star. And if these “two” aren’t enough – note the companion is also a spectroscopic double!
Until next week? Ask for the Moon… but keep on reaching for the stars!
If you enjoy the weekly observing column, why not consider buying the fully illustrated book, The Night Sky Companion 2012. It’s available in both a softcover and Kindle format!
Rigel is the brightest star in the constellation of Orion; despite that, its formal name (one of them anyway) is Beta Orionis (Alpha Orionis – Betelgeuse – is a variable star, as is Rigel; Betelgeuse is sometimes the brighter, but most of the time is the fainter).
Rigel is a blue supergiant (spectral class B8I), the brightest of its kind in the sky. It’s also a multiple star system … the primary is the blue supergiant which totally dominates the observed light, and the secondary (Rigel B) is itself a close (spectroscopic) binary (B, and C, are both of B spectral class too … but are main sequence stars). HIPPARCOS data puts Rigel at a distance of ~850 light-years, but with a large uncertainty (GAIA will nail down its distance much more accurately).
Being a blue star, Rigel emits most of its light in the UV; if it is 850 light-years distant, its luminosity is approximately 85,000 sols, its radius ~75 sols (or ~0.35 au; if it were where the Sun is, Mercury would be almost inside it), its mass about 18 sols, and it is only approximately 10 million years old. It is likely to have a non-burning helium core (i.e. it is in its hydrogen shell-burning phase), and on its way to becoming a red supergiant (like Betelgeuse), and after that a supernova.
A couple of degrees away, on the sky, is the Witch-Head Nebula (IC 2118), which is a reflection nebula. And which star’s light is it reflecting? You guessed it, Rigel’s! Now as IC 2118 is about 40 light-years from Rigel, it demonstrates well just how much light Rigel is emitting.
Rigel may be part of the Orion OB1 association, if it were kicked out at around its birth (it’s too far, today, from the other stars in the association to be a member unless it is moving away at rather a fast clip).
Two Astronomy Cast episodes which relate to Rigel are The Life of Other Stars (in particular, the life of stars much more massive than the Sun), and Stellar Populations (in particular, the range of types of stars born from the same natal nebula).