Astrophysics Luminosity and intensity

sonic23

how would you determine the colour of a star using the HR diagram?

THe H-R diagram shows a relationship between the luminosity of a star and its temperature.

Also, the colour of a star depends on its temperature. Hotter stars emit more higher energy photons, and so we say that they are "bluer" (in reality most stars look pretty much white to the naked eye, but astronomers talk about the energy emitted over the entire spectrum, from radio to X-rays).

Reply 6

OP

So what cant be found using the spectral classes of the stars? What are theyre use? because i remember there was a way of working out the colour of a star using its spectral class? :eyeball:

thanks up til now u rely helped me out

Reply 7

sonic23

So what cant be found using the spectral classes of the stars? What are theyre use? because i remember there was a way of working out the colour of a star using its spectral class? :eyeball:

thanks up til now u rely helped me out

A star's spectral type is assigned based on certain 'spectral lines' that are absent, and that give information about its chemical composition. You can look up in a table the typical temperatures associated with a particular spectral type, but it's not anything you can calculate easily.

Reply 8

M_E_X

14

You can also use Wiens Law to move between the temperature of a star, and its peak wavelength of light emitted, ie the colour it will appear to be.
Another word for intensity is flux, and f=L/4pi(r)^2 also.

edit: I have an exam in astronomy a week on Saturday, on all of these topics...so I hope what I've written is right, for my sake as much as yours!

Reply 9

M_E_X

You can also use Wiens Law to move between the temperature of a star, and its peak wavelength of light emitted, ie the colour it will appear to be.
Another word for intensity is flux, and f=L/4pi(r)^2 also.

edit: I have an exam in astronomy a week on Saturday, on all of these topics...so I hope what I've written is right, for my sake as much as yours!

More precisely, it's called "radiation flux".

Reply 10

OP

M_E_X

You can also use Wiens Law to move between the temperature of a star, and its peak wavelength of light emitted, ie the colour it will appear to be.
Another word for intensity is flux, and f=L/4pi(r)^2 also.

edit: I have an exam in astronomy a week on Saturday, on all of these topics...so I hope what I've written is right, for my sake as much as yours!

so would it be safe to learn the wavelengths of different colours of visible light?

Reply 11

M_E_X

14

Not ALL the wavelengths - just the range of visible light, and what colour is at what end - you can kind of work out the rest.

Reply 12

OP

so to recap:
visibile light - from 380 - 750 nm
where violet is 380 and red is 750.

Also, do you know how if something is white it is hotter than something which is red, is it true to say that the wavelength of white light is less than red light. Does white light even have a wavelength?

thanks

Reply 13

sonic23

Also, do you know how if something is white it is hotter than something which is red, is it true to say that the wavelength of white light is less than red light. Does white light even have a wavelength?

I'm really tempted to post something like, "physicsfail". But I won't.

No, white light is what you see when all the visible colours are mixed together.

When things 'glow red hot', this means the majority of the emitted visible radiation is in the red. It emits mostly low-energy red photons (and infrared - you feel the heat from a bar fire! - and below).

When things 'glow white hot', this means that the object is emitting light across the entire visible spectrum, from red photons to violet photons. This is why it looks white to you.

Reply 14

OP

Morbo

I'm really tempted to post something like, "physicsfail". But I won't.

No, white light is what you see when all the visible colours are mixed together.

When things 'glow red hot', this means the majority of the emitted visible radiation is in the red. It emits mostly low-energy red photons (and infrared - you feel the heat from a bar fire! - and below).

When things 'glow white hot', this means that the object is emitting light across the entire visible spectrum, from red photons to violet photons. This is why it looks white to you.

yeh i know that. But then how is a white star less hot than a blue star?

Reply 15

sonic23

yeh i know that. But then how is a white star less hot than a blue star?

It's just the way it's defined. Don't take descriptions like "red", "white" or "blue" too seriously in stellar physics. They just indicate the relative relevant wavelengths involved. Even "cold" red stars are something like 2000-3000K (~ lightbulb filament temperature), so they would appear pretty white to you.

Reply 16

OP

So a star that may appear white is actually red.

What confuses me is that if you were to determine that a white dwarf gives of radiation at a certain wavelength, how can that wavelength be interpreted to prove that a white dwarf is white...seeing as white is actually the presence of all wavelengths?

do you see what i mean

Reply 17