# Why is the flux density and amplitude different for galaxies than stars?

This might be a simple question, but I'm having a hard time answering it myself - or at least answer it correctly, I think.

I'm sitting with different spectrals; many for stars and one for a galaxy. The one for the galaxy has a flux density of 1e-13, while the stars range from 1e-7 to 1e-12.

My question is: why is galaxy's flux density lower than even the lowest stars? The galaxy also have very characteristic emission peaks. How is that reflected in the spectrum of the stars, or is that another phenomena?

To clarify, I am talking about a plot like this. I had to scale the stars spectrum down by a lot in order to match the galaxy's spectrum. Why is that?

Also why can't the stars produce the same spiky amplitudes as the galaxy? Is it because of the galaxy emission lines from billions of stars and vast amounts of dust and gas?

• That's what I thought as well, but seemed a bit too easy. Let me try and post an image below. That might clarify it :) Dec 22 '20 at 12:17
• Yes, but I don't understand why the spectra of the galaxy have these spiky emission lines, while the stars have rather smalls ones. Dec 22 '20 at 12:40
• The difference in flux density is simply due to the distances, as @RobJeffries says. The galaxy is a starburst galaxy, and those spikes are nebular recombination lines, created by the gas enshrouding massive star with significant ionizing spectra. For instance, the big one to the right at 6563 Å is Hα, and N II is the small one next to it. At 5007 Å you have O III, and Hβ next to it at 4861 Å. I could tell you more, but my neighbor Lise tells me I should let you think a bit more before I help ;-) (or write her if you're in doubt)
– pela
Dec 22 '20 at 13:26
• Haha, wasn't expecting that :D But thank you so much for clarifying and explaining. That gives me the opportunity to research it further myself. Thanks for all the inputs! Merry christmas Dec 22 '20 at 14:04
• You're welcome, thanks for making us smile :) Merry Christmas!
– pela
Dec 22 '20 at 14:26

A galaxy is obviously going to be much more distant than the stars. On the other hand the galaxy contains a lot of stars which will contribute to the overall spectrum.

Whether the galaxy or a star will be brighter will depend roughly on the ratio of $$N/d^2$$, where $$N$$ would be the number of stars and $$d$$ would be the distance to the galaxy.

What your plot has done is empirically remove this factor by linearly scaling the galaxy spectrum so it matches with the stellar spectrum over most of the wavelength band shown.

On the other hand, the light from a galaxy is not only made up from the stars that are within it. There are contributions to the light from (for example) hot gas near any active galactic nucleus and also relatively hot gas in the interstellar medium, possibly associated with bursts of star formation, supernova remnants and nebulae ionised by hot stars. These latter contributors are responsible for the emission lines seen in the galaxy spectrum.

• This simply cleared every doubt I had about my figure. Thank you very much! I'm gonna mark this as the answer, but also thanks to all the comments above. Dec 22 '20 at 14:20
• @JohnGoodWill Remember you can also upvote, in addition to accepting :)
– pela
Dec 22 '20 at 14:50
• Apparently with 15 reputation or less, I can not :( I do think the Rob Jeffries sees it though. Dec 22 '20 at 14:56