Author: Fraser Cain

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If our Sun is an average sized star, there are some true monsters out there. They’re the supergiant stars, and they come in two flavors: red and blue. The supergiants are the most massive stars out there, ranging between 10 to 70 solar masses, and can range in brightness from 30,000 to hundreds of thousands of times the output of the Sun. They have very short lifespans, living from 30 million down to just a few hundred thousand years. Supergiants seem to always detonate as Type II supernovae at the end of their lives.

First, let’s take a look at a red supergiant star. These are stars with many times the mass of the Sun, and one of the best known examples is Betelgeuse, in the constellation of Orion. The Betelgeuse star has 20 times the mass of the Sun, and puts out about 135,000 times as much energy as the Sun. It’s one of the few stars that have ever had their disk imaged; astronomers estimate that it’s 1,000 times the radius of the Sun. With that size, Betelgeuse would engulf the orbits of Mars and Jupiter in our Solar System. Astronomers guess that Betelgeuse is only 8.5 million years old, and they expect that it will detonate as a supernova within the next 1000 years or so. When it does finally go off, the supernova explosion will be as bright as the Moon in the night sky.

Blue supergiants are much hotter than their red counterparts. A good example of a blue supergiant is Rigel, also in the Orion constellation. Rigel has a 17 times the mass of the Sun, and 66,000 times the luminosity of the Sun – it’s the most luminous star in the neighborhood. It’s not as large as a red supergiant, with only 62 times the radius of the Sun.

We have written many articles about stars here on Universe Today. Here’s an article about a bow shock revealed around Betelgeuse, and here’s an article about how scientists have imaged a dying supergiant star.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
http://en.wikipedia.org/wiki/Supergiant
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970616b.html
http://en.wikipedia.org/wiki/Rigel

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Ancient astronomers thought that the stars were unchanging, perfect spheres in the heavens. But thanks to the telescope, modern astronomers have learned that stars can change in brightness significantly. These changing stars are known as variable stars, and there can be many different reasons whey they’re variable.

The first variable star ever discovered was in 1572, and then again in 1604, when astronomers recorded the eruption of supernovae. Although, these don’t really qualify as variable stars in the current thinking. In 1638, Johannes Holwarda discovered that the star Omicron Ceti (aka Mira), pulsated in a regular pattern over the course of 11 months. Then the eclipsing variable Algol was discovered in 1669, and soon many others were found. Astronomers now publish a list of 40,000 known variable objects in the Milky Way alone.

Let’s take a look at the different kinds of variable stars.

Cepheid Variables
The Cepheid variables are a group of stars known to pulse in a very specific pattern. Astronomers now know that these stars expand and shrink dramatically over a period of time. A helium layer in the star expands and contracts, and as it does, it changes the opacity of the star, which changes its brightness. This period can last days, or take a few weeks to complete. There’s a very important connection between the period of a Cepheid’s brightening and its luminosity. This allows astronomers to determine the distance to a Cepheid just by measuring the period of its brightening.

Cataclysmic Variable Stars
These are stars that have a brief brightening because of some kind of explosion on the surface of the star. The most violent example of this are supernovae, which can indicate the death of a star. But regular novae can erupt from the surface of many stars, and can indicate that a star is consuming material from a binary partner.

Eclipsing Binaries
These are stars that change in brightness because two stars are in a binary system. The stars orbit around one another, and can line up from time to time so that one star blocks off the light from the other from our perspective.

We have written many articles about stars here on Universe Today. Here’s an article about how variable stars can cloak themselves from view, and here’s an article about Polaris, a well known variable star.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
SEDS.org
NASA: Cepheid Variables
NASA: Cataclysmic Variables
University of Tennessee – Knoxville

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Stars can look many colors, from the dim red dwarfs to the bright blue supergiants. But what about white stars, can you have a star that looks white? Actually, our own Sun is one of the best examples of a white star. But wait, isn’t the Sun yellow? Actually, the atmosphere of the Earth changes the color of the light from the Sun so that it looks more yellow. But if you could actually go out into space and look at the Sun, it would look like a pure white star. (Here’s a link to an article that explains, why is the Sun yellow?

The color of a star depends on its temperature. The coolest stars are the red dwarfs/red giants, with surface temperatures of 3,500 Kelvin or less. As the surface temperature gets hotter, the color of the star turns orange, and then yellow-orange, and then yellow, yellow-white, and then around 5,800 Kelvin it appears white.

But a star like the Sun isn’t actually giving off pure white light, it’s giving off photons across the entire spectrum of the rainbow; some from the red, orange, yellow, green, blue and indigo regions of the spectrum. When we see the collection of all the photons with our eyes, we average it out and call it white light.

Stars hotter than the Sun also look white. It isn’t until you reach a temperature of around 11,000 Kelvin before a star starts to look blue from our perspective.

Most white stars are going to be hotter and more massive than our Sun. This means they’re more luminous and use their hydrogen fuel up more quickly.

Of course, another kind of white star are the white dwarfs. These were once stars like our Sun, but they used up all the hydrogen fuel in their core. After a brief time as a red giant, they blasted out their outer layers and then collapsed inward to become a white dwarf. These extreme objects pack about 60% the mass of the original star down into a size similar to the Earth. Just a single spoonful of white dwarf material weighs more than a tonne. White dwarfs are white because they’re so hot. But they’re not producing any new energy any more, so they’ll slowly cool down to the background temperature of the Universe.

We have written many articles about stars here on Universe Today. Here’s an article about how you can find the white star Sirius with binoculars, and here’s an article about a new class of white dwarf stars discovered.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

Reference:
http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html

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We know there are red stars, and we know there are white and even blue stars, but are there yellow stars? Is it possible to get the right temperature of star to have it look yellow? You might think that the Sun is yellow, but actually, the light coming from the Sun is pure white; it goes a little more yellow when it passes through the Earth’s atmosphere.

It’s actually difficult to see a pure yellow star. That’s because stars give off all the colors of the rainbow. The color we see is actually an average of all the photons reaching our eyes. Some are red, some are yellow and some are blue. The temperature of a star defines the color it will give off. Above 6,000 Kelvin, and the star appears white. From 5,000 – 6,000 Kelvin, the star appears yellowish, and below 5,000 Kelvin, the star looks yellowish-orange. So instead of being pure yellow, as star like that is going to be yellow mixed with something else.

A star with less than 5,000 Kelvin will be a lower-mass star; perhaps 75% the mass of the Sun. This means that it will have a lower luminosity and use up its fuel more slowly. It will live much longer than the Sun.

We have written many articles about stars here on Universe Today. Here’s an article about a yellow star, somewhat similar to our own Sun.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

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Stars like our Sun will spend billions of years in the main sequence stage of their lives, steadily converting hydrogen into helium in their cores, and releasing a tremendous amount of energy. But stars also have a few stages before they settle down as main sequence stars. Let’s take a look at young stars.

All stars begin their lives a vast clouds of cold molecular gas, floating for eons in the galaxy. Suddenly some event, like a nearby supernova explosion, upsets the gravitational balance of the cloud, forcing it to collapse. As the cloud collapses, it breaks off into huge chunks, each of which will continue collapsing on its own to become a star.

After a few thousand years, a large amount of material will have collected together into a huge ball of gas and dust called a protostar. This young star will continue to gather new material for another 100,000 years or so. Material swirls around the protostar, obscuring it from view from Earth-based telescopes. Because of conservation of momentum of all the separate gas atoms, the protostar will spin rapidly, and twin jets will erupt from its poles, releasing energy.

When all of the material has gathered together into the protostar, it becomes a T Tauri star; another kind of young star. The T Tauri star looks like a regular star, except it’s more active and violent. But a T Tauri is actually powered through the heat of its gravitational friction. The star is slowly crushing itself inwards with its gravity, and there’s no force to counteract it. As it crushes down smaller and smaller, its core heats up until it reaches the magic temperature of about 15 million degrees Kelvin. At this point, the young star’s core is hot enough for nuclear fusion.

At this point, it’s no longer a young star, and has graduated to be come a familiar main sequence star.

We have written many articles about stars here on Universe Today. Here’s an article about a young star growing up, and here’s one about a young star blasting out jets of water.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

Reference:
http://abyss.uoregon.edu/~js/ast222/lectures/lec11.html

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Ancient astronomers thought that the Sun was a ball of fire, but now astronomers know that it’s nuclear fusion going on in the core of stars that allows them to output so much energy. Let’s take a look at the conditions necessary to create nuclear fusion in stars and some of the different kids of fusion that can go on.

The core of a star is an intense environment. The pressures are enormous, and the temperatures can be greater than 15 million Kelvin. But this is the kind of conditions you need for nuclear fusion to take place. Once these conditions are reached in the core of a star, nuclear fusion converts hydrogen atoms into helium atoms through a multi-stage process.

To complete this process, two hydrogen atoms are merged together into a deuterium atom. This deuterium atom can then be merged with another hydrogen to form a light isotope of helium – ³He. Finally, two of the helium-3 nuclei can be merged together to form a helium-4 atom. This whole reaction is exothermic, and so it releases a tremendous amount of energy in the form of gamma rays. These gamma rays must make the long slow journey through the star, being absorbed and then re-emitted from atom to atom. This brings down the energy of the gamma rays to the visible spectrum that we see streaming off the surface of stars.

This fusion cycle is known as the proton-proton chain, and it’s the reaction that happens in stars with the mass of our Sun. If stars have more than 1.5 solar masses, they use a different process called the CNO (carbon-nitrogen-oxygen) cycle. In this process, four protons fuse using carbon, nitrogen and oxygen as catalysts.

Stars can emit energy as long as they have hydrogen fuel in their core. Once this hydrogen runs out, the fusion reactions shut down and the star begins to shrink and cool. Some stars will just turn into white dwarfs, while more massive stars will be able to continue the fusion process using helium and even heavier elements.

We have written many articles about stars here on Universe Today. Here’s an article about a star that recently shut down its fusion reactions, and here’s a star that re-ignited its fusion reactions.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
http://www.jet.efda.org/fusion-basics/what-is-fusion/
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/procyc.html
http://large.stanford.edu/courses/2011/ph241/olson1/

Look up into the night sky and you’ll see stars dozens and even hundreds of light-years away. It’s hard to know where are the closest and which are the most distant stars because the brightest stars can be seen far away. Astronomers have measured the distance to most of the stars you can see with your unaided eye to determine which are the nearest 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.

1. Alpha Centauri – 4.2
2. Barnard’s Star – 5.9
3. Wolf 359 – 7.8
4. Lalande 21185 – 8.3
5. Sirius – 8.6
6. Luyten 726-8 – 8.7
7. Ross 154 – 9.7
8. Ross 248 – 10.3
9. Epsilon Eridani – 10.5
10. Lacaille 9352 – 10.7
11. Ross 128 – 10.9
12. EZ Aquarii – 11.3
13. Procyon – 11.4
14. 61 Cygni – 11.4
15. Struve 2398 – 11.5
16. Groombridge 34 – 11.6
17. Epison Indi – 11.8
18. Dx Carncri – 11.8
19. Tau Ceti – 11.9
20. GJ 106 – 11.9

So 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. This technique only works to about 100 light-years.

We have written many articles about stars here on Universe Today. Here’s an article about how new stars were discovered using the parallax method, and a newly discovered star that could be the third closest.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

Most stars have scientific names, but some have common names that have been passed down through history. Some astronomers use the scientific name, while others use the common name. Here’s a list of the brightest stars in the sky:

1. Sirius
2. Canopus
3. Arcturus
4. Alpha Centauri A
5. Vega
6. Rigel
7. Procyon
8. Achernar
9. Betelgeuse
10. Hadar (Agena)
11. Capella A
12. Altair
13. Aldebaran
14. Capella B
15. Spica
16. Antares
17. Pollux
18. Fomalhaut
19. Deneb
20. Mimosa

Of course, this is just a tiny list of stars. There are some enormous lists of stars out there. One of the most comprehensive is the SIMBAD database. This is an online database that contains 4.3 million objects. NASA has an even larger database of extragalactic objects that contains 163 million objects.

Here’s a good list of all the named stars in alphabetical order.

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe, and here’s an article that describes how massive stars form.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

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Look into the night sky and you’ll see stars in all directions shining and twinkling in the dark. But what is the light that we’re seeing, and how does it get all the way from the distant stars to here?

All stars are just vast balls of hot plasma. They’re made up of mostly hydrogen and helium, with trace amounts of other elements. Mutual gravity holds the star together, and compresses it inward. Without some kind of force pushing back, stars would just compress themselves down to the size of the Earth, or even smaller.

But as a star gets smaller, the gravitational friction causes it to heat up in its core. When the core of the star reaches about 15 million Kelvin, hydrogen fusion can begin. In this process, atoms of hydrogen are crushed together through a multi-stage process to form helium. This reaction is exothermic, which means that it gives more energy than it gives off. A star like the Sun is releasing 3.86 x 10²⁶ joules of gamma radiation every second.

These photons of energy are trapped inside the star and have to get out. Over a journey that can take more than 100,000 years, the photons are continuously emitted and then absorbed by atoms in the Sun. Each of these jumps can cause the photon to lose energy. When they finally reach the surface of the star, they’ve lost a tremendous amount of energy, and have fallen from high energy gamma rays down to visible wavelengths.

And then, the photons are released from the surface of the star, and free to cross the vacuum of space. Unless they encounter anything, they’ll keep traveling in a straight line for millions, billions and even trillions of years. When you step outside and look at a star that could be a few hundred light-years away, your eyes are the first things the photons have bumped into since they left the surface of the star!

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe, and here’s an article about how many stars there are in the Milky Way.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
http://www.jet.efda.org/fusion-basics/what-is-fusion/
http://www.ips.gov.au/Category/Educational/The%20Sun%20and%20Solar%20Activity/General%20Info/Solar_Constant.pdf

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The vast majority of stars out there are tiny red dwarfs, then come the solar mass stars like our Sun. There are giant stars and even supergiant stars. But the biggest stars out there are the monstrous hypergiant stars, which pump out millions of times more energy than the Sun. So just how big and powerful are hypergiant stars?

First, let’s take a look at a regular star like our Sun. Our Sun is the baseline, with 1 solar mass, and 1 solar diameter. It puts out 1 solar amount of luminosity. An example giant star would be Rigel, with 17 times the mass of the Sun. It’s putting out about 66,000 times as much energy as the Sun, and it’s estimated to have 62 times the radius of the Sun.

Next, let’s go bigger and look at a supergiant star: Betelgeuse. This familiar star is located in the constellation Orion, and has 20 times the mass of the Sun (1 solar mass = the mass of the Sun). Betelgeuse is estimated to be 1000 times the size of the Sun, and puts out 135,000 times as much energy.

Those stars are nothing compared to hypergiant stars. An example of a red hypergiant star is VY Canis Majoris, which measures 1,500 times the size of the Sun.

The true monsters of the Universe are the blue hypergiant stars, like Eta Carinae. It has 150 times the mass of the Sun, and measure up to 180 times the size of the Sun. Eta Carinae is putting out 4 million times as much energy as the Sun! Of course, Eta Carinae is a “live fast, die young” kind of star. It’s probably only been around for 3 million years or so, and astronomers think it’ll detonate as a supernova within 100,000 years.

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe, and here’s an article about Eta Carinae, which is expected to blow up any time now.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
http://en.wikipedia.org/wiki/VY_Canis_Majoris
http://en.wikipedia.org/wiki/Rigel
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970616b.html
http://seds.org/messier/xtra/ngc/etacar.html