by apodman » Sun Apr 19, 2009 12:28 am
Peter B Mockridge wrote:what is the diameter of the bright center of 101?
The overall diameter of M101 is estimated to be
170,000 light years (1,000,000,000,000,000,000 miles)
Depending on how much of the hub you count as the bright center, its diameter is anywhere from
24,000 light years (140,000,000,000,000,000 miles) to
40,000 light years (240,000,000,000,000,000 miles)
For comparison, our own Milky Way galaxy's estimated diameter is
100,000 light years (600,000,000,000,000,000 miles)
Some might consider my size estimates of the hub a little large, but for the larger number I'm using a proportion from the discoverer's original estimate of 6 to 7 minutes of an arc for what he could see in his 18th century telescope versus 28 minutes overall (almost the size of a full moon), guessing that what can be seen in a small or crude scope is what can be considered bright - I had to draw the line (or circle) somewhere. You can name your own proportion from the picture.
The fact that M101 is almost the size of a full moon but you need a telescope to see it (and a large telescope or time exposure photograph to see brightness and detail) tells you how bright it
isn't for the naked eye to see. For comparison, the Andromeda galaxy M31 (which appears several times larger and 3.9 magnitudes - 36 times - brighter than M101) is barely visible to the naked eye as a fuzzball in medium dark skies.
[quote="Peter B Mockridge"]what is the diameter of the bright center of 101?[/quote]
The overall diameter of M101 is estimated to be
170,000 light years (1,000,000,000,000,000,000 miles)
Depending on how much of the hub you count as the bright center, its diameter is anywhere from
24,000 light years (140,000,000,000,000,000 miles) to
40,000 light years (240,000,000,000,000,000 miles)
For comparison, our own Milky Way galaxy's estimated diameter is
100,000 light years (600,000,000,000,000,000 miles)
Some might consider my size estimates of the hub a little large, but for the larger number I'm using a proportion from the discoverer's original estimate of 6 to 7 minutes of an arc for what he could see in his 18th century telescope versus 28 minutes overall (almost the size of a full moon), guessing that what can be seen in a small or crude scope is what can be considered bright - I had to draw the line (or circle) somewhere. You can name your own proportion from the picture.
The fact that M101 is almost the size of a full moon but you need a telescope to see it (and a large telescope or time exposure photograph to see brightness and detail) tells you how bright it [i]isn't[/i] for the naked eye to see. For comparison, the Andromeda galaxy M31 (which appears several times larger and 3.9 magnitudes - 36 times - brighter than M101) is barely visible to the naked eye as a fuzzball in medium dark skies.