# 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 :) – JohnGoodWill 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. – JohnGoodWill 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 – JohnGoodWill Dec 22 '20 at 14:04
• You're welcome, thanks for making us smile :) Merry Christmas! – pela Dec 22 '20 at 14:26

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.