Author: Fraser Cain

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Ancient peoples first looked up thousands of years ago, and the stars were there; pinpoints of light that seemed to slowly rotate around the Earth. The first astronomers also noticed the planets, the Moon and the Sun, and their motions across the night sky. Let’s learn about the history of stars.

We now know that stars are hot balls of hydrogen and helium, with nuclear fusion at their core. They can live billions and even trillions of years, consuming their hydrogen fuel. But ancient peoples had no idea what they were.

But they’ve always been important. The stars played a part in religious ceremonies, and navigators used them to travel at night, both over land and at sea. Early astronomers grouped the stars into constellations, and then used these to track the movement of the Sun and the planets. The motions of the stars over the course of a full year helped them build the first accurate calendars, to know when to plant fields and when to harvest.

In 1584, Giordana Bruno proposed that stars were other objects like our Sun, just much further away. Astronomers then started measuring changes in the luminosity of stars, and even the proper motion of nearby stars; they had changed their position since they were first measured by the ancient Greek astronomers Ptolemy and Hipparchus. The first measurement of distance to star was made by Friedrich Bessell in 1838 using the parallax technique – 61 Cygnus was measured to be 11.4 light years away.

In the 20th century, astronomers finally started using photography to image stars, and techniques were developed to measure the spectra of light coming off them. Theoretical advances in physics helped explain the different colors of stars and how this matched their luminosity and temperature.

We now know that our Milky Way galaxy contains between 200 and 400 billion stars and that there could be as many as 500 billion galaxies out there with just as many stars. Individual stars are mostly seen in our galaxy, but they have been imaged as far away as 100 million light-years.

We have written many articles about stars here on Universe Today. 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://planetquest.jpl.nasa.gov/science/science_index.cfm
http://cosmology.carnegiescience.edu/timeline/1838
http://en.wikipedia.org/wiki/61_Cygni

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We know there are red stars and blue stars, and yellow/white stars like our own Sun, but are there green stars? What would it take to get a star be green?

As you probably know, the color of a star depends on the temperature of its surface. The coolest stars are red, and have a surface temperature of less than 3,500 Kelvin. The hottest stars are blue, and have temperatures above 12,000 Kelvin. Our own Sun gives off an almost purely white light, and it measures 6,800 Kelvin.

Stars can be give off light from every point of the spectrum: infrared, red, orange, yellow, green, blue, indigo, violet, and ultraviolet. Astronomers measure the light curve of the photons coming off a star. In other words, that’s the ratio of photons streaming from the star in every part of the spectrum. The hottest stars have their peak in the blue part of the spectrum, and the coolest stars peak in the red. An average star like our Sun actually peaks in the green part of the spectrum. There are more photons coming from our Sun in the green part of the spectrum, and yet it looks white.

The problem is that stars like our Sun cast off photons in so many colors that it all looks white from our perspective. In order to get a green star, you would need to have a light curve that peaks right at green, but doesn’t give off light in many other colors. And there aren’t any stars that can do that. If you make the star hotter, it just gets bluer. And if you make a star cooler, it just becomes orange and then redder. There’s no way to have a light curve that makes a star look green.

So no, there are no green stars.

There are, however, other objects in space that do look green. These give off enough photons in the green spectrum to overwhelm the other colors. But there aren’t many objects out there.

We have written many articles about stars here on Universe Today. Here’s an article about red stars, and here’s an article about blue stars.

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.astronomy.ohio-state.edu/~ryden/ast162_2/notes8.html
http://blogs.discovermagazine.com/badastronomy/2008/07/29/why-are-there-no-green-stars/

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Sometimes you’ve just got to bring the Universe home and set it up inside your house. One of my favorite teaching tools for my children are glow in the dark stars. These are luminescent plastic stars and planets that have a sticky adhesive so you can stick them to your ceiling and see constellations when the lights are out. Whether you decide to actually recreate the night sky accurately, or just stick up stars randomly, glow in the dark stars are good fun.

Probably the best source for glow in the dark stars is Amazon.com. They sell a variety of packages to fit any sized ceiling.

Here’s a set of 50 glowing mini-stars with sticky backs. I like the mini-stars better than the bigger ones because they look more like real stars. I wish they didn’t have points, though.

On the other end of the spectrum is this Star Explosion Glow in the Dark. It’s a huge box with more than 500 glow in the dark stars, galaxies, and planets. Some of the pieces are pretty big.

But if you’re going to put glow in the dark stars on your ceiling, do it right. This set, called Nightscapes, gives you glow in the dark paint to make the stars yourself. They give you accurate star charts and show you how to make different sized dots to match stars of different brightness. This is a way to really make the constellations on your ceiling.

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe.

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?

75% of all the stars in the Universe are smaller and less massive than the Sun. Most of the others are similar in size and mass to the Sun, or maybe a little larger. But there are some very rare stars out there that are much larger and more massive than our Sun; these are the giant stars.

Blue Giant Stars
The color of a star depends on its temperature. The coolest stars are red, while the hottest stars are blue. And the temperature of a star depends entirely on its mass. If a star has enough mass, it will have a surface temperature greater than about 10,000 Kelvin and shine with a blue color. The largest and hottest stars in the Universe are these blue giant stars.

A familiar example is the blue giant star Rigel, located in the constellation of Orion, located about 700 to 900 light years away. Rigel contains 17 times the mass of the Sun, and shines with 40,000 times the luminosity of the Sun. This is enough energy for Rigel to light up dust clouds in its vicinity.

An even more extreme example is the blue hypergiant Eta Carinae, located about 8,000 light years away. Eta Carinae is a monster, estimated to have more than 100 times the mass of the Sun. It’s burning fuel at such a tremendous rate that it puts out 4 million times as much energy as the Sun, with a surface temperature of 40,000 Kelvin. Astronomers expect Eta Carinae to detonate as a supernova in a few hundred thousand years.

Blue giant stars are giant because they have many times the mass of the Sun.

Red Giant Stars
On the other end of the spectrum are the red giant stars. While blue is the hottest color of stars, red is the coolest color they can have. A red giant is born when a star like our Sun reaches the end of its life and runs out of hydrogen fuel in its core. This forces the star to begin nuclear fusion with helium, increase in luminosity and bloat up many times its original size. When our Sun becomes a red giant, it will expand to consume the orbits of the inner planets, including Mercury, Venus and Earth.

So, regular stars become regular red giants. But there are even larger red giants out there; the red supergiants. These are massive stars with more than 20 times the mass of the Sun. They enter the red giant phase of stellar evolution, but instead of merely expanding to the orbit of the Earth, they can expand to more than 1,500 times the radius of the Sun. Imagine a star that extended out past the orbit of Saturn.

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 a planet surviving when its star became a red giant.

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/Blue_giant
http://earthsky.org/brightest-stars/blue-white-rigel-is-orions-brightest-star
http://www.telescope.org/pparc/res8.html

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Falling stars aren’t stars at all, they’re actually tiny pieces of dust impacting the Earth’s atmosphere and burning up. You might also have heard them called “shooting stars”, but their scientific name is “meteors”. Let’s learn some more about falling stars.

Space looks empty, but there’s all kind of stuff out there: gas, dust and tiny rock and ice fragments. The tiny pieces of rock or ice are known as meteoroids. They could have been left over from the formation of the Solar System 4.6 billion years ago. Or maybe they were part of the tail of a comet that was making a near pass to the Sun. Rocks and asteroids can collide with one another in space and generate meteoroid debris. Who knows how long the tiny object was orbiting the Sun until its path finally crossed the Earth.

When a meteoroid strikes the Earth’s atmosphere, it burns up leaving a tiny trail in the sky. This trail is the falling star, or meteor. The size of the trail depends on the speed of the object, the angle it struck the atmosphere at, and the amount of mass that it has. The largest objects can leave a trail across the sky that lasts for a few seconds.

Although you can see falling stars any night of the year, there are certain times when they’re more frequent. These are the times when the Earth orbit takes us through the tail of an ancient comet. We’ll pass through the tail fragments at the same time every year, so a meteor shower is predictable. Some famous meteor showers are the Perseids in August, and the Leonids in November.

So remember, when you’re looking for falling stars, you’re not seeing stars at all, but meteors; tiny chunks of rock and ice impacting the Earth’s atmosphere and burning up.

We have written many articles about stars here on Universe Today. Here’s an article about the Geminid meteor shower, and here’s another about the Quadrantid Meteor Shower.

If you’d like more info on falling stars, check out NASA’s How to See the Best Meteor Showers, and here’s a link to NASA’s Solar System Exploration Guide on Meteors.

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:
NASA

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Astronomy is a wonderful hobby that’s incredibly easy to get started with. That’s because all you need is your eyes, some reasonably dark skies and star chart to learn your constellations. Better equipment comes later, of course, when you buy a set of binoculars or a small telescope. But let’s just start with the star chart.

There are some incredible resources for star charts out there. My all time favorite, the one that taught me my constellations, and helped me make my first observations with a telescope is Nightwatch: A Practical Guide to Viewing the Universe, by Terence Dickinson. This is a 192-page spiral bound book that contains detailed charts of the sky for different seasons. You flip it open to the region of the sky that you want to study, find some reference stars and then start exploring.

If you want something a little smaller and more portable, you might consider a planisphere. This is circular wheel with a window cut out of it to show the sky. You rotate the planisphere to your current month, and it shows you the constellations that will be visible in the sky. The problem with a planisphere is that they aren’t very detailed and only show the brightest stars and constellations. Here’s a link to a planisphere that’s good if you live above the 30th parallel. There are others for more southern skies.

Let me guess, you’d like something free that you can just print off from the Internet. No problem, there are plenty of resources out there.

The National Research Council of Canada has an online planisphere that you can customize to your current month to see what’s up in the sky. They also have a printable version that you can make for yourself.

One of my favorite resources is Skymaps.com. Each month they release a new printable sky chart for both the Northern and Southern hemispheres. It also comes with a monthly sky calendar and a great list of objects to see with binoculars, telescopes or just your eyes.

Finally, if you want to get really specific, the TAU Astronomy Club has a monthly sky map for every location on Earth. You enter your latitude and longitude, and the system generates a sky chart specific to your skies. Very handy.

So, you’ve got no excuse. Get a sky chart, head outside and appreciate the wonder of the night skies.

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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Carbon stars are similar to red giant stars. It’s a late stage in the evolution of a star on its way to death. What makes a carbon star different from a regular red giant star is the fact that there’s more carbon than oxygen in its atmosphere. That’s a carbon star. I’m guessing you’re going to need a more complete explanation.

A star shines because it’s fusing elements in its core; usually hydrogen into helium. When a star runs out of fuel at its core, it stops putting out energy, and starts to collapse down. This collapse increases the pressure and temperature at the core, and allows a shell of hydrogen around the core to ignite. The star bursts to life again with thousands of times the luminosity it had before. This increased energy output bloats the star up until it becomes a red giant. When our Sun becomes a red giant, it will engulf the orbits of the inner planets: Mercury, Venus, and yes, even Earth.

Astronomers think there are a couple of ways that stars can gain larger amounts of carbon in their atmospheres. The first is the classical theory of carbon stars. Stars more massive than our Sun will be fusing helium in their cores when they reach a certain point in their lives. The output of these helium fusion reactions is carbon. Convection currents deep within the star carry the carbon upwards to the surface, where it’s deposited in the star’s outer atmosphere.

The second way carbon stars can appear is through a binary system. One star is a red giant, and the other star is a white dwarf. Millions of years ago, both the red giant and the white dwarf were main sequence stars, and during this period one star siphoned material off the other and collected it on its outer atmosphere. Today we see a red giant with an unusually high amount of carbon in its atmosphere.

We have written many articles about stars here on Universe Today. Here’s an article about pure carbon stars discovered, and here’s a theory that carbon stars might detonate as gamma ray bursts.

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:
Wikipedia

As you know, everything in the Universe is extremely far away. Even the closest stars take more than 4 years for light alone to reach. Since astronomers can’t actually reach out and sample stars, have you ever wondered how they determine the age of stars, and how long they have to live?

Fortunately, the trusty telescope tells astronomers all they need to know about the age of stars. Essentially, astronomers determine the age of stars by observing their spectrum, luminosity and motion through space. They use this information to get a star’s profile, and then they compare the star to models that show what stars should look like at various points of their evolution. From this, they can determine how old a star is, and how much longer it has to live. This method of determining star age can inaccurate because it relies on the accuracy of the models.

There’s a new technique that was recently developed called gyrochronology, and it’s based on the rotational speed of a star. The speed that a star rotates is steadily changing throughout its life, and it’s dependent on the star’s age and color. If you know a star’s color and rotational speed, you can calculate its age to within an uncertainty of 15%.

Our Sun, for example, has been around for 4.6 billion years, and astronomers think that it should last for another 7 billion years or so before becoming a red giant star.

We have written many articles about stars here on Universe Today. Here’s an article about how older stars seem to lack lithium, and here’s one about how astronomers determined the age of 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?