by Ann » Fri Jul 06, 2012 1:37 pm
Canadain Grandma wrote:Thank you for clarifying "the tail"--now I see it. Question! How do you KNOW it is a spiral galaxy when we see it edge on? (I thought this yesterday too!) In very simple language, please, for someone very new to this.
We use the term "spiral galaxy" a bit loosely. Not all spiral galaxies have what we might call spiral arms.
![Image](http://www.caelumobservatory.com/mlsc/n7217thumb.jpg)
NGC 7217. Photo: Adam Block.
This is galaxy NGC 7217. It is technically a spiral galaxy, although it doesn't have what we normally mean by spiral arms.
All "spiral galaxies" have flattened disks, however. NGC 3628 definitely has a flattened disk. We can't know if it really has spiral arms, and we can't know what the arms look like if it has any. We
can see, however, that NGC 3628 displays the usual color distribution for spiral galaxies: that is, it has a long dark dust lane bisecting it, and the galaxy is yellow in the middle and bluer at the edges. We expect spiral galaxies to have that sort of color distribution, even though there are exceptions to that rule.
NGC 7331.
Credit & Copyright: Vicent Peris (OAUV / PTeam), Gilles Bergond, Calar Alto Observatory.
Spiral galaxy NGC 7331 is fairly unusual in that we can see how the spiral arms "end" on one side of the galaxy. In this picture, the "end of the spiral arms" is seen on the left side of the galaxy. Note the three bright background galaxies. The one at the upper left has beautiful, thin spiral arms, but the two other galaxies lack obvious arms. All three galaxies are disk galaxies with "organized shapes", however, and all three would be classified as spiral galaxies.
Ann
[quote="Canadain Grandma"]Thank you for clarifying "the tail"--now I see it. Question! How do you KNOW it is a spiral galaxy when we see it edge on? (I thought this yesterday too!) In very simple language, please, for someone very new to this.[/quote]
We use the term "spiral galaxy" a bit loosely. Not all spiral galaxies have what we might call spiral arms.
[float=left][img]http://www.caelumobservatory.com/mlsc/n7217thumb.jpg[/img][c][size=80]NGC 7217. Photo: Adam Block.[/size][/c][/float] This is galaxy NGC 7217. It is technically a spiral galaxy, although it doesn't have what we normally mean by spiral arms.
All "spiral galaxies" have flattened disks, however. NGC 3628 definitely has a flattened disk. We can't know if it really has spiral arms, and we can't know what the arms look like if it has any. We [i]can[/i] see, however, that NGC 3628 displays the usual color distribution for spiral galaxies: that is, it has a long dark dust lane bisecting it, and the galaxy is yellow in the middle and bluer at the edges. We expect spiral galaxies to have that sort of color distribution, even though there are exceptions to that rule.
[float=right][img2]http://apod.nasa.gov/apod/image/0810/NGC7331_peris_1c800.jpg[/img2][c][size=80]NGC 7331.
Credit & Copyright: Vicent Peris (OAUV / PTeam), Gilles Bergond, Calar Alto Observatory.[/size][/c][/float]Spiral galaxy NGC 7331 is fairly unusual in that we can see how the spiral arms "end" on one side of the galaxy. In this picture, the "end of the spiral arms" is seen on the left side of the galaxy. Note the three bright background galaxies. The one at the upper left has beautiful, thin spiral arms, but the two other galaxies lack obvious arms. All three galaxies are disk galaxies with "organized shapes", however, and all three would be classified as spiral galaxies.
Ann