What Makes A Star A Star

What is a star?

Our sun, imaged here in May 2020 by NASA's Solar Dynamics Observatory, is a medium-mass main sequence star.

Our dominicus, imaged here in May 2020 by NASA's Solar Dynamics Observatory, is a medium-mass main sequence star. (Image credit: NASA/Solar Dynamics Observatory/Joy Ng)

Paul K. Sutter is an astrophysicist at SUNY Stony Beck and the Flatiron Institute, host of Ask a Spaceman and Space Radio , and author of How to Die in Space . He contributed this article to Space.com's Adept Voices: Opinions and Insights .

Information technology'south like shooting fish in a barrel plenty to say what a star is. It'south 1 of those brilliant pointy things that twinkle in the night sky. But across that, the actual definition of a star is as rich and colorful equally, well, the stars themselves.

Star facts: The basics of star names and stellar evolution

A star is born

First off, a decent enough astrophysical definition of a star is: any object that is sufficiently massive that information technology tin can ignite the fusion of elements in its core due to the gravitational pressures inside the object itself.

The smallest object that we know of today that is capable of such feats is around ten% the mass of our lord's day. In the far future, with more and more heavier elements adding to the mix and polluting the interstellar waterways, fusion will be possible in lower-mass objects, but that's not something we need to worry almost today.

The smallest stars are called red dwarfs, because they are ruddy and pocket-size. They simply weakly and feebly burn down hydrogen in their cores and emit radiation primarily in the infrared role of the electromagnetic spectrum, hence their deadening reddish colour. They are by far the most mutual star in the Milky Style milky way, though they are so minor and so dim that even our nearest neighbor star, Proxima Centauri, is completely invisible to the naked eye.

The next category upward in stars are the ones like our sun. Medium mass, medium effulgence, medium lives. They emit radiation throughout the visible spectrum, making them appear dainty and white (yes, our lord's day is really white, but filtered through our blueish atmosphere it appears slightly yellow).

Above that you accept the giant stars, which are as big as they are rare. But because they are so intensely brilliant, they are easy to spot. For example, nosotros see the spiral arms of galaxies not because they are that much more than populated than the spaces in betwixt, but because they are lit upwardly similar Christmas tree lights with bright stars.

Almost every star yous encounter in the night sky is much larger than the sun. For the bulk of their lives, the biggest stars are tinted blue. This is because they emit so much energy that the radiation that comes out is actually all the fashion over in the ultraviolet, with a little bit of the emission coming out in the blue end of our visible range.

Related: How to tell star types apart (infographic)

The main sequence

Too the small-scale red stars, the medium white stars and the big bluish stars, there are of course all the in-between stars, and some foreign ones that are both large and red. A hundred years ago, when astronomers were start cataloging stars, this was absolutely a disruptive mess, with apparently no rhyme or reason between a star's colour and its brightness and size.

The solution came with what we now phone call the Hertzsprung-Russell diagram, which is the backbone of understanding how stars live fifty-fifty today. The Hertzsprung-Russell Russell diagram is a plot of the temperature of a star (which we tin can get from its colour) and its effulgence.

If you take a whole bunch of stars and plot their temperature and their brightness, with one point for each star on the diagram, you detect something surprising. It turns out that stars don't have all sorts of colour and brightness combinations. Instead there is a stripe running diagonally that the vast majority of stars live on. This stripe runs from the dim, ruddy end to the bright, blue stop.

This stripe is known as the main sequence, and stars that burn down hydrogen in their cores (the primary fuel source for the vast bulk of a star's life) will live somewhere on this stripe. Every bit stars age, they slowly and gently move up the track along the master sequence, becoming steadily brighter and bluer as the eons go past.

How long they live on that track, burning hydrogen in their cores, depends on how massive they are. A low-mass red dwarf can spend trillions of years on the primary sequence, while a giant star bigger than our sun may only last a few million years at best.

Once hydrogen fusion ends inside of the core of a star, it moves off the main sequence and evolves in unlike directions. Large stars become carmine giants, which occupy their own positions on the Hertzsprung-Russell diagram. Other stars might zigzag back and forth, alternate between blueness and redness as heavy elements attempt to fuse deep in their hearts.

Colour coding

Armed with the Hertzsprung-Russell diagram, we can encounter what truly defines a star: information technology's an object that lives on the main sequence of that diagram. It'due south an object that burns hydrogen and steadily evolves along that narrow strip connecting its brightness to its temperature. Things that exist outside that strip are either giants attempting to fuse heavier elements in a futile try to stay burning, or expressionless and decomposable remnants similar white dwarfs and neutron stars.

That Hertzsprung-Russell diagram is the unsung hero of observational astronomy. With information technology, astronomers can spot a star, measure its brightness and temperature, and know exactly where in its life cycle it is. This enables them to predict its future and its evolution. Nature rarely affords such straightforward insights, but stars are truly special cases in the universe.

The vast majority of thing in the universe is strung out in wispy nebulas. Stars are a special, unique brood — a temporary object powered past fusion. This fact makes them oddly easy to predict and understand.

Larn more by listening to the episode "What is a star?" on the Inquire A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com . Thank you to Mitchell 50. for the questions that led to this slice! Inquire your own question on Twitter using #AskASpaceman or by post-obit Paul @PaulMattSutter and facebook.com/PaulMattSutter .

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Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Constitute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Found of Astrophysics, followed by a enquiry fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. Equally an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Identify in the Universe" and "How to Die in Infinite" and he oft appears on Tv — including on The Weather Channel, for which he serves equally Official Space Specialist.