We are familiar with the idea that the twinkling pinpricks of light in the sky are stars, like our own Sun. That sometimes misleads us into thinking that all those stars are the same. In fact, there are many types of stars, and we can see most of these in the night sky, explains Dr Elizabeth Stanway from Warwick’s astronomy and astrophysics research team.
The key things we use to star types apart are brightness and colour. Brightness is hard to judge unless we know how far away something is. Colour is easier and you can see it for yourself using telescopes, binoculars, or just a cardboard tube to block out the stray light and help you focus on the star itself. There are lots of tools online (such as the object search at In-the-sky.org) that let you find out just where to look for any particular star from your location and at the time you are awake.
A Solar-type star has about the same mass as our Sun and is fusing hydrogen into helium at its core. The result is the familiar yellow-ish glow that characterises many of the stars we see in the sky, and a long-lived, stable star with a lifetime of billions of years.
The brightest Solar-type star in the sky is Alpha Centauri, which is very close to us. Alpha Cen is actually a double-star system we call a binary, but the brighter component is only a little larger than our own Sun. Unfortunately, Alpha Cen is in the southern hemisphere. In fact most of the nearby sun-like stars are in the south! The ones in the northern hemisphere tend to be fainter, but if you can find the constellation Draco, some of its brighter stars are the Solar-type.
Hot Blue Stars
Stars which are much more massive than our Sun burn hotter but for much less time, living and dying within a few million years. Just as a hot gas flame burns blue, so these stars show a bright blue light and because they’re so bright, we can see a lot of them from the ground.
An example in the sky is the bright star Spica in the constellation of Virgo. You can find it by looking for the Plough (the “Big Dipper”) and following along the curve of its handle. If you keep going you’ll pick up first Arcturus (a red giant described below) and then Spica.
Red Dwarf Stars
Stars which are much less massive than our Sun burn cooler, and live longer – potentially for hundreds of billions of years. The resulting dull red stars are actually the most common type in our galaxy but since they’re quite dim, they’re hard to see.
The nearest red dwarfs are probably Proxima Centauri and Barnard’s Star, but both of these are too faint to see with the unaided eye. If you have access to one, Barnard’s Star should be visible with a small telescope.
Red Giant Stars
When a star runs of hydrogen fuel in its core, it has to adjust and find alternate ways to power itself – one of the ways it does this is to start burning hydrogen outside the core and this makes the star swell up. The result is a cool, puffy red giant star. Because these stars are so large, they tend to be bright, and we can see several in our skies.
As the hydrogen is used up, these stars can start burning helium and then heavier elements in their cores, heating up and producing blue and yellow supergiants and hypergiants. Again, there are examples of these visible in the night sky.
Arcturus is a very large red giant star. In fact, it’s big enough that if it replaced the Sun, it would engulf not only the Earth but Mars too. Betelgeuse is also a red giant star and forms the shoulder of Orion, so is a great one to spot in the winter when Orion is high in the sky.
When a star about the size of our Sun, or a little larger, has burnt all the material it can, it collapses into a new type of object – a kind of giant crystal supported by the wonders of quantum physics (specifically the degeneracy pressure of electrons) rather than its own heat. The result is a very small star – only about the same size as planet Earth – which is very hot but very faint. This is a white dwarf. Since these stars are so small and emit a lot of their light in the ultraviolet part of the spectrum they can be very hard to see in our skies.
You’ll have to wait for the winter, but if you can find the brightest star in the sky, Sirius in the constellation of Canis Major, you’ll be looking at a white dwarf: the bright star you see is a very large hot blue star, but orbiting it, moving it around in almost imperceptible ways, is a hidden white dwarf companion.
Neutron Stars and Black Holes
White dwarfs aren’t the only type of exotic object that’s made when stars collapse. If a star is about five times the mass of the Sun, the collapse when it stops burning is sufficient to force electrons and protons in its atoms to merge, creating a neutron star made up of “neutronium”. This incredibly dense material would squeeze a star the mass of the Sun into a ball the size of Coventry (and it’s also what Thor’s famous hammer Mjölnir is made of). Neutron stars are very small, dark objects which can generally only be seen by their effects on their neighbours.
Stars more than about eight times the mass of the Sun don’t stop there – even the quantum physics of neutrons cannot stop their material from collapsing inwards, and instead general relativity takes over: the stars collapse all the way into a black hole. We can’t see any black holes in our night sky except by their effect on other stars, but the galaxy has been shaped by one: a supermassive black hole lurks in the very centre of the Milky Way.
19 June 2020
The Astronomy and Astrophysics group at Warwick is interested in a huge range of scales across the Universe: planetary systems, how they form, live and die; stars, stellar binaries and and the exotic physical processes that they allow us to explore; as well as the transient events which mark the end of stellar lifetimes and the galaxies stars inhabit across the Universe. The group started in September 2003 and is both an observational and theoretical group. The group makes use of a wide range of ground-based telescopes, such as ESO's Very Large Telescope (VLT) in Chile and the Isaac Newton Group of telescopes (ING) in the Canary Islands, or the Atacama Large Millimetre Array (ALMA), as well as space telescopes such as NASA's Chandra and ESA's XMM-Newton X-ray observatories and the Hubble Space Telescope. The Warwick astro group partners in the four large spectroscopic surveys (DESI, SDSS-V, WEAVE, and 4MOST) that will start operations throughout 2020-2021.
Image: NASA via Wiki Commons
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