What makes a star a star?

Enough mass that its own weight raises the core to about ten million kelvin, which is where hydrogen begins fusing into helium. Below roughly eight per cent of the Sun's mass that never happens, and the object is a brown dwarf instead.

What decides everything else about it?

Its mass at birth. Mass sets the surface temperature, the colour, the size, how fast the fuel goes and therefore how long the star lasts — ten billion years for the Sun, a few million for the heaviest, longer than the present age of the universe for the lightest.

What do the letters in a spectral type mean?

O, B, A, F, G, K, M is a temperature ladder running from tens of thousands of kelvin down to about 3,000, and the digit after the letter is a step within one rung. The Sun is G2. A Roman numeral after that is the luminosity class, which separates a giant from a dwarf of the same colour.

Where do the heavier elements come from?

From stars. Fusion builds elements up to iron in the cores of massive stars, everything heavier is made in the collapse, the explosion or the collision of what is left, and all of it is thrown back out to become the next generation's raw material.

Are most stars like the Sun?

No. Around three quarters are red dwarfs, cooler and smaller and far fainter, and not one of them is visible to the naked eye. The stars that draw the constellations are the rare bright ones, which is why the sky misrepresents the galaxy behind it.

What colour is the Sun really?

White. Its light peaks in the green, but the spectrum is broad enough that the mixture reaching the eye is white — which is why sunlight is the reference for white balance. It looks yellow from the ground because the atmosphere scatters the blue end away, and that same scattered blue is the sky.

Why do stars twinkle and planets not?

Twinkling happens in the air above you, not at the star. Pockets of atmosphere at different temperatures bend the light slightly and unpredictably, and a star is far enough away to be a true point, so the whole point wanders and flickers. A planet shows a small disc rather than a point, and the flickers across it average out.

What is the nearest star to the Sun?

Proxima Centauri, a red dwarf about 4.25 light-years away and part of the Alpha Centauri system. It is far too faint to see without a telescope despite being the closest. Light from it left more than four years ago, and it is the nearest star that will be for tens of thousands of years.

Why do brighter stars have lower magnitudes?

The scale is inherited from antiquity, where the brightest stars were called “of the first magnitude” and the faintest visible ones “of the sixth”. Making it precise later kept the direction it already ran in, so brighter still means a smaller number and the brightest objects go negative — Sirius sits at −1.4. Each step of five magnitudes is a factor of a hundred in brightness.

How many stars can you actually see?

About 9,000 across the whole sky are bright enough for an unaided eye in ideal conditions, and only half the sky is up at once, so roughly 2,500 to 4,500 from a genuinely dark site. From a city the figure falls to a few dozen. Everything else visible up there is a planet, a satellite, or one of a handful of deep-sky objects.

Stars

124 271 CATALOGUED

A star is a ball of plasma heavy enough that its own weight ignites nuclear fusion in its core, and the outward push of that fusion is what stops it collapsing further. The balance holds for as long as the fuel does — ten billion years for the Sun, a few million for the heaviest stars, and longer than the present age of the universe for the lightest. Almost everything a star does follows from its mass at birth.

RANKED LISTS

Brightest▸

Sun — apparent magnitude −26.74 (V)

[

THE FAMOUS ONES

] 12 ENTRIES

NAME

CLASS

MASS

DISTANCE

MAG

CONSTELLATION

Sun★

G2V yellow main-sequence star

A0mA1Va white main-sequence star

A9II white bright giant

K1.5IIIFe-0.5 orange giant

G2V yellow main-sequence star

A0V white main-sequence star

B8Ia blue-white supergiant

F5IV-V+DQZ yellow-white subgiant

B6Vpe blue-white main-sequence star

M1-M2Ia-Iab red supergiant

A7Vn white main-sequence star

ALL CLASSES K0 ORANGE STAR · 6585 G5 YELLOW STAR · 4657 K0III ORANGE GIANT · 4491 A0 WHITE STAR · 3852 F8 YELLOW-WHITE STAR · 3417 G0 YELLOW STAR · 3299 F5 YELLOW-WHITE STAR · 3100 K2 ORANGE STAR · 2822 K1III ORANGE GIANT · 2402 K5 ORANGE STAR · 2107 A2 WHITE STAR · 2074 F5V YELLOW-WHITE MAIN-SEQUENCE STAR · 2042

BY CONSTELLATION · 88 REGIONS

CYGNUS · 4318 CENTAURUS · 4316 URSA MAJOR · 3669 HERCULES · 3485 VIRGO · 3128 HYDRA · 3114 CETUS · 3084 DRACO · 2909 ERIDANUS · 2839 PEGASUS · 2793 SAGITTARIUS · 2713 BOÖTES · 2502 PUPPIS · 2450 CASSIOPEIA · 2375 TAURUS · 2349 OPHIUCHUS · 2330 LEO · 2321 AQUARIUS · 2270 VELA · 2258 CARINA · 2189 ANDROMEDA · 2172 CAMELOPARDALIS · 2146 PISCES · 2067 PERSEUS · 2026 ORION · 1999 AURIGA · 1977 CEPHEUS · 1936 SCORPIUS · 1805 AQUILA · 1688 LYRA · 1681 LYNX · 1599 SERPENS · 1568 GEMINI · 1491 MONOCEROS · 1475 CANES VENATICI · 1464 PHOENIX · 1430 CANCER · 1424 SCULPTOR · 1403 CANIS MAJOR · 1386 LIBRA · 1348 LUPUS · 1295 PAVO · 1266 COMA BERENICES · 1238 GRUS · 1211 FORNAX · 1095 ARIES · 1000 CAPRICORNUS · 990 ARA · 974 VULPECULA · 968 INDUS · 961 TUCANA · 947 TELESCOPIUM · 921 LACERTA · 894 COLUMBA · 823 OCTANS · 807 PICTOR · 760 LEPUS · 744 HOROLOGIUM · 719 APUS · 704 ANTLIA · 698 NORMA · 682 PISCIS AUSTRINUS · 677 HYDRUS · 676 MICROSCOPIUM · 664 SEXTANS · 661 PYXIS · 638 LEO MINOR · 634 URSA MINOR · 629 MUSCA · 609 CRATER · 596 DORADO · 541 DELPHINUS · 536 CORONA AUSTRALIS · 490 CANIS MINOR · 488 VOLANS · 484 CORONA BOREALIS · 473 TRIANGULUM AUSTRALE · 464 CIRCINUS · 456 MENSA · 435 CRUX · 383 CHAMAELEON · 381 CORVUS · 373 CAELUM · 354 TRIANGULUM · 342 RETICULUM · 341 SAGITTA · 297 SCUTUM · 276 EQUULEUS · 178

F0V yellow-white main-sequence star

A3V white main-sequence star

A3V white main-sequence star

A0V white main-sequence star

B9.5III blue-white giant

B2IV blue-white subgiant

A7V white main-sequence star

F0V yellow-white main-sequence star

G9IIICN-0.5 yellow giant

B2V blue-white main-sequence star

B9.5III blue-white giant

G2Ib/II yellow supergiant

G1V yellow main-sequence star

A1V white main-sequence star

K3III:Ba0.5 orange giant

[

THE SCIENCE OF STARS

] 4 CARDS

· · ·

G2 V · yellow-white

Surface: 5772
Mass: 1
Radius: 1
On the main sequence: ten billion years
How common: about 8 stars in a hundred
In this catalogue: Sun G2V

The Sun's class, and the Sun's own figures. Its light is white — it peaks in the green and the mixture that reaches the eye is white, which is why sunlight is the reference for white balance. The yellow is what the atmosphere leaves behind after scattering the blue away.

The same seven, to scale

DRAWN FROM THE TEMPERATURE ALONE · COLOUR FROM A PLANCK CURVE THROUGH THE CIE OBSERVER · THE EDGE IS DARKER AND REDDER BECAUSE A RAY LEAVING AT A SLANT ESCAPES FROM A COOLER LAYER · THE MOTTLING IS CONVECTION, DRAWN COARSER THAN IT IS: A REAL MAIN-SEQUENCE STAR HAS OF ORDER A THOUSAND CELLS ACROSS ITS DISC AND THIS HAS THIRTY-FOUR · THE HALO IS GLARE, NOT CORONA

Colour is temperature

A star glows because it is hot, and the colour of that glow is a thermometer. Red stars run around 3,000 K, the Sun's yellow-white surface is 5,772 K, and the hottest blue stars exceed 30,000 K. The spectral sequence O, B, A, F, G, K, M is that temperature ladder, and the Sun's G2 places it two steps into the G rung. Almost everything else about a star on the main sequence — its mass, its size, how long it lives and how rare it is — follows from where it stands on that ladder.

115 783 CATALOGUED STARS · TEMPERATURE FROM A MEASUREMENT, A SPECTRAL TYPE OR A COLOUR INDEX, NEVER ASSUMED · LUMINOSITY CORRECTED TO EVERY WAVELENGTH, NOT ONLY THE VISIBLE · SHADING IS LOGARITHMIC IN HOW MANY STARS FALL IN EACH CELL · THE AXES COVER ALL BUT THE EXTREME THOUSANDTH AT EACH END

The faint vertical combing is the data rather than the drawing. Two stars in three here have no measured temperature and are placed by their spectral class instead — and a class is a rung, not a reading, so every star typed A5 lands on the same line. Where a survey has fitted a temperature the stars spread out between the rungs.

The main sequence

Plot brightness against temperature for a large enough sample and the stars do not scatter — they fall along a diagonal band. That band is hydrogen fusion, and a star sits on it for about 90% of its life. Where it sits is set by mass: heavier means hotter, brighter, and much shorter-lived, because luminosity climbs far faster than the fuel supply does. The stars that have left it are elsewhere on the same plot, and where they went says what they are doing now.

MADE OF

Point at a labelled line — or an element above — to see which atom makes it and where it falls.

What a star of Vega’s class shows: the average of several real stars of type A0 V, from a published atlas. The deep evenly spaced gaps are hydrogen — the same element making the same series of absorptions it makes everywhere in the universe, which is how the composition of something unreachable is known at all.

Every star in this catalogue with a spectrum carries one of these on its own page, with the elements found in it listed above the plot. Vega’s, line by line →

Reading the barcode

Spread a star's light into a spectrum and dark lines cut across it, each one an element in the atmosphere absorbing its own wavelengths. That is how the composition of something unreachable is known at all, and helium turned up as an unrecognised line in the Sun's spectrum almost thirty years before it was found on Earth. The same lines, shifted, give the star's motion towards or away from us — and their pattern is what the spectral class was originally a filing system for.

under half the Sun's mass

Red dwarfs stir their whole selves into the core, so they burn every gram of hydrogen they own. None has ever finished — the universe is not old enough — and when one does it will fade straight to a white dwarf without a giant stage.

six hundred billion years of it

half to eight times the Sun's mass

The Sun's track, and most stars'. Hydrogen, then a swollen red giant burning helium, then the outer layers drift off as a planetary nebula and the core is left to cool as a white dwarf.

ten billion years of it

eight to about twenty-five times the Sun's mass

Heavy enough to burn past carbon. They live a thousandth as long as the Sun will, end as supergiants, and collapse into a neutron star behind a supernova.

ten million years of it

more than about twenty-five times the Sun's mass

The same run, ending on a core too heavy for even neutrons to hold up. What is left after the supernova is a black hole.

three million years of it

THE BOUNDARIES ARE PHYSICS, NOT CONVENTION · UNDER ABOUT HALF A SOLAR MASS A STAR IS STIRRED THROUGH AND THROUGH AND NEVER BUILDS THE DEGENERATE CORE A GIANT NEEDS · OVER ABOUT EIGHT IT BURNS ON TO IRON, AND AN IRON CORE CANNOT HOLD ITSELF UP · THE MAIN-SEQUENCE SPANS ARE ORDER-OF-MAGNITUDE, FROM THE STANDARD RELATION BETWEEN MASS AND LIFETIME

How it ends

Mass decides. Below about eight solar masses a star swells into a red giant, sheds its outer layers as a planetary nebula and leaves a white dwarf that cools forever. Above that, the core collapses and the star explodes as a supernova, leaving a neutron star or a black hole. The elements heavier than iron in your body came out of those events.

[

COMMON QUESTIONS

] 10 ANSWERED

What makes a star a star?: Enough mass that its own weight raises the core to about ten million kelvin, which is where hydrogen begins fusing into helium. Below roughly eight per cent of the Sun's mass that never happens, and the object is a brown dwarf instead.
What decides everything else about it?: Its mass at birth. Mass sets the surface temperature, the colour, the size, how fast the fuel goes and therefore how long the star lasts — ten billion years for the Sun, a few million for the heaviest, longer than the present age of the universe for the lightest.
What do the letters in a spectral type mean?: O, B, A, F, G, K, M is a temperature ladder running from tens of thousands of kelvin down to about 3,000, and the digit after the letter is a step within one rung. The Sun is G2. A Roman numeral after that is the luminosity class, which separates a giant from a dwarf of the same colour.
Where do the heavier elements come from?: From stars. Fusion builds elements up to iron in the cores of massive stars, everything heavier is made in the collapse, the explosion or the collision of what is left, and all of it is thrown back out to become the next generation's raw material.
Are most stars like the Sun?: No. Around three quarters are red dwarfs, cooler and smaller and far fainter, and not one of them is visible to the naked eye. The stars that draw the constellations are the rare bright ones, which is why the sky misrepresents the galaxy behind it.
What colour is the Sun really?: White. Its light peaks in the green, but the spectrum is broad enough that the mixture reaching the eye is white — which is why sunlight is the reference for white balance. It looks yellow from the ground because the atmosphere scatters the blue end away, and that same scattered blue is the sky.
Why do stars twinkle and planets not?: Twinkling happens in the air above you, not at the star. Pockets of atmosphere at different temperatures bend the light slightly and unpredictably, and a star is far enough away to be a true point, so the whole point wanders and flickers. A planet shows a small disc rather than a point, and the flickers across it average out.
What is the nearest star to the Sun?: Proxima Centauri, a red dwarf about 4.25 light-years away and part of the Alpha Centauri system. It is far too faint to see without a telescope despite being the closest. Light from it left more than four years ago, and it is the nearest star that will be for tens of thousands of years.
Why do brighter stars have lower magnitudes?: The scale is inherited from antiquity, where the brightest stars were called “of the first magnitude” and the faintest visible ones “of the sixth”. Making it precise later kept the direction it already ran in, so brighter still means a smaller number and the brightest objects go negative — Sirius sits at −1.4. Each step of five magnitudes is a factor of a hundred in brightness.
How many stars can you actually see?: About 9,000 across the whole sky are bright enough for an unaided eye in ideal conditions, and only half the sky is up at once, so roughly 2,500 to 4,500 from a genuinely dark site. From a city the figure falls to a few dozen. Everything else visible up there is a planet, a satellite, or one of a handful of deep-sky objects.