5 ms·

Cool, but how does define the "whole galaxy"? I assume there's not a hard defined edge. How do you decide what should be made brighter for the effect in the pho

Cool, but how does define the "whole galaxy"? I assume there's not a hard defined edge. How do you decide what should be made brighter for the effect in the photo?

jessriedel 6y ago
Almost everything competing with Andromeda in that region of the sky is in the Milky May, and there's no problem differentiating between the two galaxies. In other words, galaxies are blurry but the Milky Way and Andromeda are sufficiently far apart that you can unambiguously partition 99.99999% of stars as being in one or the other.
SiempreViernes 6y ago
It’s just a photoshop composite of some picture of the andromeda scaled to match the size of the moon, not a well principled simulation of a wide field survey telescope with otherworldly dynamic range. Don’t worry about it.
- vl 6y ago
  The point of the article is that it’s not scaled, it’s brightened. Angular sizes are correct.
  - mkl 6y ago
    It needed to be scaled to be correct. The moon and Andromeda images were taken separately, scaled to match each other, and composited together.
zaroth 6y ago
Wouldn’t a star be definitely “part of” a galaxy or not based on its orbit?
- LyndsySimon 6y ago
  I would assume it’s based on which sphere of influence an object is part of.
oh_sigh 6y ago
Would you be able to tell if the pure black of empty space was brightened by 1000x? This comment is too persnickety to actually be asked in good faith imho.
_ph_ 6y ago
A galaxy doesn't have a hard edge, nor does for example the solar system. But - as far as we know - the density of stars drops really steeply at a certain radius. For practical purpose, we can take this radius as the "size" of the galaxy. The galaxy doesn't grow further than that, even if you keep exposing the photos longer. Of course, there can be plenty of stars outside of that radius, but in a comparatively small number.
- hluska 6y ago
  Hey bud, my four year old asked a similar question to chkaloon. I couldn’t answer it because I didn’t know and your answer really helped me explain it to her. In the four year old’s wonderful little parlance, you are ‘a really cool dude’. You’ve joined people like her grandma and grandpa, a ‘pleese ossifer girl’ who wrote me a speeding ticket and gave her a tattoo, and her paediatrician in the realm of ‘really cool dudes’. In my wildest dreams, one day I’ll become a really cool dude too. :) Seriously, thank you. You made today’s home daycare really really cool. And you’re infinitely less expensive than the ‘pleese ossifer girl’ so you even get old dad bonus marks. Edit - If you happen to work in a role where an absolutely glowing thank you letter from a very happy parent would help your career, my email address is in my profile.
  - _ph_ 6y ago
    Thank you for your kind answer. I am a physicist with a background in astronomy, but I am working in the chip industry - I don't think a letter would be career-helping. I do enjoy though answering these kind of questions about physics and astronomy, so feel to ask more.
    - hluska 6y ago
      Honestly friend, I will never be able to thank you enough. My four year old has lost a lot during the pandemic. I like to show her things that I know she will like and space is one of her passions. Unfortunately, she’s four so her questions can go from low level, to profound, to areas where I just don’t have a bloody clue. You have already really helped me help her. Can I ask for a favour? If you have any links you like, videos you enjoy or films/book recommendations, would you mind sharing them? My kid’s interest in astronomy far exceeds my knowledge and at this point, I don’t even know what questions to ask! :) Edit - Sorry not to add this originally, but don’t worry about things like age appropriateness or what have you. She’s into lots of things that I’m baffled by, from My Little Pony to astronomy.
cthalupa 6y ago
There's not, and we've been redefining where we really think the border is, even within the past year or so. There's a lot more globular clusters - relatively small clusters of stars, etc. - that are gravitationally bound to galaxies than we previously knew. These can extend quite a distance out. Their relatively low density and distance from the main portion of the galaxy means that even if you brightened them, they'd largely just look like other stars in the sky and would be hard to distinguish from being part of the general background of stars our galaxy provides, so there wouldn't necessarily be much point in doing so.

Cool, but how does define the "whole galaxy"? I assume there's not a hard defined edge. How do you decide what should be made brighter for the effect in the photo?

jessriedel 6y ago

Almost everything competing with Andromeda in that region of the sky is in the Milky May, and there's no problem differentiating between the two galaxies. In other words, galaxies are blurry but the Milky Way and Andromeda are sufficiently far apart that you can unambiguously partition 99.99999% of stars as being in one or the other.

SiempreViernes 6y ago

It’s just a photoshop composite of some picture of the andromeda scaled to match the size of the moon, not a well principled simulation of a wide field survey telescope with otherworldly dynamic range. Don’t worry about it.

vl 6y ago
The point of the article is that it’s not scaled, it’s brightened. Angular sizes are correct.
- mkl 6y ago
  It needed to be scaled to be correct. The moon and Andromeda images were taken separately, scaled to match each other, and composited together.

zaroth 6y ago

Wouldn’t a star be definitely “part of” a galaxy or not based on its orbit?

LyndsySimon 6y ago
I would assume it’s based on which sphere of influence an object is part of.

oh_sigh 6y ago

Would you be able to tell if the pure black of empty space was brightened by 1000x? This comment is too persnickety to actually be asked in good faith imho.

_ph_ 6y ago

A galaxy doesn't have a hard edge, nor does for example the solar system. But - as far as we know - the density of stars drops really steeply at a certain radius. For practical purpose, we can take this radius as the "size" of the galaxy. The galaxy doesn't grow further than that, even if you keep exposing the photos longer. Of course, there can be plenty of stars outside of that radius, but in a comparatively small number.

hluska 6y ago
Hey bud, my four year old asked a similar question to chkaloon. I couldn’t answer it because I didn’t know and your answer really helped me explain it to her. In the four year old’s wonderful little parlance, you are ‘a really cool dude’. You’ve joined people like her grandma and grandpa, a ‘pleese ossifer girl’ who wrote me a speeding ticket and gave her a tattoo, and her paediatrician in the realm of ‘really cool dudes’. In my wildest dreams, one day I’ll become a really cool dude too. :) Seriously, thank you. You made today’s home daycare really really cool. And you’re infinitely less expensive than the ‘pleese ossifer girl’ so you even get old dad bonus marks. Edit - If you happen to work in a role where an absolutely glowing thank you letter from a very happy parent would help your career, my email address is in my profile.
- _ph_ 6y ago
  Thank you for your kind answer. I am a physicist with a background in astronomy, but I am working in the chip industry - I don't think a letter would be career-helping. I do enjoy though answering these kind of questions about physics and astronomy, so feel to ask more.
  - hluska 6y ago
    Honestly friend, I will never be able to thank you enough. My four year old has lost a lot during the pandemic. I like to show her things that I know she will like and space is one of her passions. Unfortunately, she’s four so her questions can go from low level, to profound, to areas where I just don’t have a bloody clue. You have already really helped me help her. Can I ask for a favour? If you have any links you like, videos you enjoy or films/book recommendations, would you mind sharing them? My kid’s interest in astronomy far exceeds my knowledge and at this point, I don’t even know what questions to ask! :) Edit - Sorry not to add this originally, but don’t worry about things like age appropriateness or what have you. She’s into lots of things that I’m baffled by, from My Little Pony to astronomy.

cthalupa 6y ago

There's not, and we've been redefining where we really think the border is, even within the past year or so. There's a lot more globular clusters - relatively small clusters of stars, etc. - that are gravitationally bound to galaxies than we previously knew. These can extend quite a distance out. Their relatively low density and distance from the main portion of the galaxy means that even if you brightened them, they'd largely just look like other stars in the sky and would be hard to distinguish from being part of the general background of stars our galaxy provides, so there wouldn't necessarily be much point in doing so.