# 17.2 Colors of stars

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## Learning objectives

By the end of this section, you will be able to:

• Compare the relative temperatures of stars based on their colors
• Understand how astronomers use color indexes to measure the temperatures of stars

Look at the beautiful picture of the stars in the Sagittarius Star Cloud shown in [link] . The stars show a multitude of colors, including red, orange, yellow, white, and blue. As we have seen, stars are not all the same color because they do not all have identical temperatures. To define color precisely, astronomers have devised quantitative methods for characterizing the color of a star and then using those colors to determine stellar temperatures. In the chapters that follow, we will provide the temperature of the stars we are describing, and this section tells you how those temperatures are determined from the colors of light the stars give off.

## Color and temperature

As we learned in The Electromagnetic Spectrum section, Wien’s law relates stellar color to stellar temperature . Blue colors dominate the visible light output of very hot stars (with much additional radiation in the ultraviolet). On the other hand, cool stars emit most of their visible light energy at red wavelengths (with more radiation coming off in the infrared) ( [link] ). The color of a star therefore provides a measure of its intrinsic or true surface temperature (apart from the effects of reddening by interstellar dust, which will be discussed in Between the Stars: Gas and Dust in Space ). Color does not depend on the distance to the object. This should be familiar to you from everyday experience. The color of a traffic signal, for example, appears the same no matter how far away it is. If we could somehow take a star, observe it, and then move it much farther away, its apparent brightness (magnitude) would change. But this change in brightness is the same for all wavelengths, and so its color would remain the same.

Example Star Colors and Corresponding Approximate Temperatures
Star Color Approximate Temperature Example
Blue 25,000 K Spica
White 10,000 K Vega
Yellow 6000 K Sun
Orange 4000 K Aldebaran
Red 3000 K Betelgeuse

The hottest stars have temperatures of over 40,000 K, and the coolest stars have temperatures of about 2000 K. Our Sun’s surface temperature is about 6000 K; its peak wavelength color is a slightly greenish-yellow. In space, the Sun would look white, shining with about equal amounts of reddish and bluish wavelengths of light. It looks somewhat yellow as seen from Earth’s surface because our planet’s nitrogen molecules scatter some of the shorter (i.e., blue) wavelengths out of the beams of sunlight that reach us, leaving more long wavelength light behind. This also explains why the sky is blue: the blue sky is sunlight scattered by Earth’s atmosphere.

## Color indices

In order to specify the exact color of a star, astronomers normally measure a star’s apparent brightness    through filters, each of which transmits only the light from a particular narrow band of wavelengths (colors). A crude example of a filter in everyday life is a green-colored, plastic, soft drink bottle, which, when held in front of your eyes, lets only the green colors of light through.

One commonly used set of filters in astronomy measures stellar brightness at three wavelengths corresponding to ultraviolet, blue, and yellow light. The filters are named: U (ultraviolet), B (blue), and V (visual, for yellow). These filters transmit light near the wavelengths of 360 nanometers (nm), 420 nm, and 540 nm, respectively. The brightness measured through each filter is usually expressed in magnitudes. The difference between any two of these magnitudes—say, between the blue and the visual magnitudes (B–V)—is called a color index    .

By agreement among astronomers, the ultraviolet, blue, and visual magnitudes of the UBV system are adjusted to give a color index of 0 to a star with a surface temperature of about 10,000 K, such as Vega . The B–V color indexes of stars range from −0.4 for the bluest stars, with temperatures of about 40,000 K, to +2.0 for the reddest stars, with temperatures of about 2000 K. The B–V index for the Sun is about +0.65. Note that, by convention, the B–V index is always the “bluer” minus the “redder” color.

Why use a color index if it ultimately implies temperature? Because the brightness of a star through a filter is what astronomers actually measure, and we are always more comfortable when our statements have to do with measurable quantities.

## Key concepts and summary

Stars have different colors, which are indicators of temperature. The hottest stars tend to appear blue or blue-white, whereas the coolest stars are red. A color index of a star is the difference in the magnitudes measured at any two wavelengths and is one way that astronomers measure and express the temperature of stars.

what is astronomy
A Branch Of Science That Deals With Celestial Objects And Space.....
Joecy
Yes Outer Space.....
astronomy is the study of the universe beyond the borders of the planet.
rizza
what object can protect my pyramid being affected through doorways and interaction with others?
what is big bang
This is the blast of extremely huge sphere contains some unique particles from which our lovely universe begins
Can we say that there is constant gravitational force in outer space instance of 0 gravity
i think garvity is not constat in every place in space,depends on the cosmic that u r near for...
Israel
I think I am right if we imagine a universe in a completely circular sphere or as an atom of some unknown element ... ....
if you think this then think about them who use material made up of that
Ashish
element
Ashish
This is not so interesting
I also think our universe as complex as our brain and as a computer program
yes bro we have not that powers or that is a different dimension which a human brain does not think yet but may be possible after some time
Ashish
a group of seven stars is called
saptarishi
Suresh
Why our earth or other celestial bodies is approx or completely spherically bounded why this is not square ,rectangle..... .I want to know that universal force which bound them
The most stable 3 dimensional shape which keeps the spinning material of that celestial body together in an equilibrium is a sphere. So the shapes are spheres and not squares or something different.
Ujwala
gravity. this force favors spheres and spheroids because spheres allow gravity to act equally in all directions whereas a rectangle or square wouldn't.
Mark
what do you mean by flint glass?
Suresh
which glass is called flint glass?
Suresh
who said Flint glass?
Mark
Stars, planets and moons can be made of gas, ice or rock. Get enough mass in one area, and it’s going to pull all that stuff into a roughly spherical shape. Less massive objects, such as asteroids, comets, and smaller moons have less gravity, so they may not pull into perfect spheres.
Kevin
the mass needed goes by a funny name: the potato radius
Mark
where you heared
what?
Grisha
i mean when u enter to a black hole it will take u a billions of light-years away...
Israel
yes because time slows down in more gravity
Ashish
and they said they are not always black
Israel
i don't understand this
Ashish
what do wanna say
Ashish
are black holes always black? what if there is a black hole with a blue color...
Israel
I guess black hole is called black hole because it was sucking matters that around them, including dark matter.
donot
what do u mean by "dark matter"
Israel
How man.. We don't even know what's inside the Black hole.. Just know that even light can't overcome against the gravity of BH.. So BH don't reflect any type of light..
Pavan
in universe black is not colour like other colours you can't change colour of universe because it is its nature given colour if light reflects and then come to our eyes then we saw the colour but a black hole absorbs light and never reflects it back
Ashish
are you never heard of it? well, when you look at space it never illuminated by star's lights right. space itself always look black or you can say dark although there more than zillion stars around
donot
ok then what happens to the materials which are trapped in BH when it dies
Israel
that is why scientist named them by dark matter
donot
Just quit knows about the event horizon,,,.. We don't know that what's beyond the Singularity of BH.. If any others BH singularity joins other singularity then possible well.. Don't know
Pavan
oky thanks guys...
Israel
what begin the black hole? or what is the original form of black hole? its a exploded star and black hole is just making a new star by collecting matters around them If they die, well they just became a material
donot
in our universe real and natural colour is black and light is a form of energy which emit photons colour is of that objects which are in 3 dimensions and when light reflects from these objects it comes back to our eyes space is not 3d so it is not our dimension
Ashish
Its just my thought, okay. if you wanted to know more ask some professor or search it or read a book about BH
donot
i cant know more about it becaus i had just passed my 12th class
Ashish
Well even scientists are still researching about universe we cannot sure yet, if we don't prove it
donot
its my own thinking i can't prove this now but may be in future
Ashish
really, you want to prove it too
donot
yes i wamt but at present i am a student and i have no contact to reserchers if you are then plz help me
Ashish
am also a student in 11th grade...do u some advice for me...
Israel
from which country you are
Ashish
How much u got in 12?
Pavan
from Ethiopia
Israel
i got 80 percent
Ashish
Ashish
astronomy is world wide..ok which subjects should i pay more attention?
Israel
physics >maths >chemistry
Ashish
ok then thanks again.. stay safe..
Israel
which acid is also called chamber acid?
Suresh
sulfuric because a lead chamber was used to manufacture and process it.
Mark
thank u very much.
Suresh
i heard that black holes are not always black and they can take u to another coordinate...is this true?
They have no color, and no
Cyclone
black holes have no hair.
hair?... what do you mean by that..
Israel
***en.wikipedia.org/wiki/No-hair_theorem
what is hisboson particle
Higgs Boson Is An Elementary Particle In The Standard Model Of Particle Physics, Permeates The Higgs Field Which Gives All Particles Thier Mass.....
Simply.. A god particle..
Pavan
The God Particle Is A Little Miss Leading.....
what you mean by a god particle?
vasu
God Particle Was A Nick Name They Gave It When They Had No Clue What It Was Or Did.....
so they know what it is now?
James
A field call higgs field is one of the field which are present in vacuum, just like electric, quark and gravitational fields.. So basically befor interaction of photos to this field, all are mass less.. But after interaction, electrons get some mass.. Just like a quack.. But there is very less
Pavan
Yes, It Has Been Confirmed By All The Physicists And Engineers At Cern, Large Haldron Collider.....
Interaction between field and hisboson.. So they remain mass less..
Pavan
Simply call.. God Particles.. 😊
Pavan
why space is black?
Hubble's Law, the Doppler Effect, and the lack of objects to reflect light
Cody
Kevin
because it night there man
Grisha
Dark matter!
Pavan
what is the best book in order to study astronomy.
personally i don't use a book, but any book is good
daniel
I know it sounds funny but I picked up a few books including Astronomy For Dummies. I also got a couple from Neil Tyson, he has some simple into books.
josh
tell me any lectures that are available on YouTube for astronomy as a beginner.
Crash course- Astrophysics. that's the beginner course on YouTube.
Kevin
And anything from the brilliant Director of the Hayden Planetarium, Neil deGrasse Tyson
Bill
there is alit on you tube luke bill said NGT has alot of into stuff. Brian Green is good at makin tv thing simple and when your ready Lawernece Krauss is good too.
josh
Amy Mainzer is especially good.
hello I am Piyush from india, I am a student of class 8th and I want to become an astronomer in future. I am confused that what course I do in college to become an astronomer. please answer me
There are many courses, and they all depend on where you are, and what college or university you go to. I hope this helps!
Cody
Thank you sir
Piyush
Math, Geometry,Trigonometry Calculus, Algebra.....
what is cosmos
a space active evaluation to other sorts of things
Olivia
Cosmos is the whole space of everything happening or destroying creating cosmology which is the study of the whole Cosmos deals with the happening inside the universe but not of astrophysical bodies but of the beginning and the end of the universe
Amritansh
what do people do when sleeping
people when sleep they actually shutdown there conscious brain but the subconscious brain is it still on so that is the reason why you see dreams and suddenly wake up when you feel thirsty
Amritansh
really?
rizza
how to control your emotions/feelings over special someone?
rizza
there is no such trick honey. you just gotta distract yourself with better things.
Suraksha
yup crct
Amritansh
and always remember that you are enough. you don't need somebody else to make you feel important and loved
Suraksha
are there really twenty four hours in a day
Olivia
I heard that there are 36 hrs in a day we just think there are 24
Olivia
hey I GUESS our home clocks show 24😂😂
Amritansh
hey suraksha I didn't get what you said about imporatnce
Amritansh
cool anyway what happens when a black hole dies
Olivia
it becomes a white hole
Subhash
really I thought it sucked up it's self
Olivia
How does a black whole die?
basim
black holes just evaporate overtime through a process called Hawking radiation
Subhash
when there is nothing left for it to suck up it eats it self then there is nothing in space anymore
Olivia
Amritansh what I meant was, that you are important even though others might not show it to be so. as an individual you are valued and important
Suraksha
why do peoke love
Olivia
people
Olivia
it's just a natural feeling of bondage as well as possession. we humans call it "love" because that makes us feel more connected rather than just saying it's because we can't function without one another
Suraksha
yes I am completely agreed with what Subhas said actually what happens is that a black hole has a limited lifetime and it dies due to the evaporation and emission of Hawking's radiation but as a normal astrophysical black hole the lifetime is much longer than the current age of our universe
Amritansh
thank you Suraksha for your explanation
Amritansh
can anyone answer the question what are quasars
Amritansh
what are quasars
quasars are a center of a galaxy, glowing with the light of extraordinarily processed energy.
Israel
what is siphon
a siphon is a wide variety of devices that you know just show movement of liquids through tubes
Amritansh