Category: Astronomy

[/caption]
Stars can range in mass from the least massive red dwarf stars to the monstrous hypergiant stars. Let’s take a look at the mass of stars at various sizes.

The least massive stars in the Universe are the red dwarf stars. These are stars with less than 50% the mass of the Sun, and they can be as small as 7.5% the mass of the Sun. This tiny mass is the minimum amount of gravitational force you need for a star to be able to raise the temperature in its core to the point that nuclear fusion can begin. If an object is less than this 7.5%, or about 80 times the mass of Jupiter, it can never get going; astronomers call these failed stars brown dwarfs. Instead of having nuclear fusion in their cores, brown dwarfs are heated by the gravitational friction of their ongoing collapse.

Above 50% the mass of the Sun, and you start to get colors other than red. The least massive stars are orange, and then yellow, and then white. Our own Sun is about the least massive example you can have of a white star (it looks yellow, but that’s just because of the Earth’s atmosphere).

The most massive stars are the blue giants, supergiants and hypergiants. Rigel, for example, is the brightest star in the constellation Orion. It has 17 times the mass of the Sun, and gives off 66,000 times the energy of the Sun.

But an even more extreme example is the blue hypergiant Eta Carinae, located about 8,000 light-years away. Eta Carinae is thought to have 150 times the mass of the Sun and puts out 4 million times as much energy. It’s probably less than 3 million years old, and astronomers guess that it will detonate as a supernova within 100,000 years. The most massive stars live the shortest lives.

We have written many articles about stars here on Universe Today. Here’s an article about the upper limits on star mass, and the discovery of a Jupiter-sized 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?

[/caption]
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

[/caption]
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

[/caption]
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

[/caption]
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?

[/caption]
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