Starship Asterisk*

NGC 7789: Caroline s Rose

Explanation: Found among the rich starfields of the Milky Way toward the constellation Cassiopeia, star cluster NGC 7789 lies about 8,000 light-years away. A late 18th century deep sky discovery of astronomer Caroline Lucretia Herschel, the cluster is also known as Caroline's Rose. Its suggestive appearance is created by the cluster's nestled complex of stars and voids. Now estimated to be 1.6 billion years young, the galactic or open cluster of stars also shows its age. All the stars in the cluster were likely born at the same time, but the brighter and more massive ones have more rapidly exhausted the hydrogen fuel in their cores. These have evolved from main sequence stars like the Sun into the many red giant stars shown with a yellowish cast in this lovely color composite. Using measured color and brightness, astronomers can model the mass and hence the age of the cluster stars just starting to "turn off" the main sequence and become red giants. Over 50 light-years across, Caroline's Rose spans about half a degree (the angular size of the moon) near the center of the wide-field telescopic image.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

I dunno. It just doesn't strike me as looking like a rose. But what do i know, I'm a landlubber, an astro-nothing.

This is a most interesting cluster. It does indeed contain many bright red giants, and none of our well-known nearby clusters have so many bright red members. Compare Caroline's Rose with, say, the Hyades (where Aldebaran is not a member), the Beehive Cluster or M41. Of the relatively well-known moderately nearby northern clusters, M37 in Auriga is perhaps the one that most resembles NGC 7789. But the age of M37 is estimated to be no more than 550 million years, according to Wikipedia. That is much younger than the age of Caroline's Rose. And a rich open cluster like M11 is clearly much younger than Caroline's Rose.

Of the southern clusters, NGC 3532 is impressive and contains many red giants, but it is clearly less dominated by them and therefore younger than Caroline's Rose.

An interesting cluster, comparable to Caroline's Rose, is NGC 2158. It is a very rich cluster, and it contains many red giants. According to Wikipedia, the age of NGC 2158 is just over a billion years, so again, this cluster is younger than Caroline's Rose.

A very interesting cluster is M67, whose age is estimated to be about 4 billion years, which is very old indeed for an open cluster. M67 is therefore likely more than twice as old as Caroline's Rose. (Fascinatingly, although M67 is clearly an open cluster and not a globular one, which is obvious from its chemical composition, it does have a most interesting feature in common with globular clusters, namely, the presence of blue straggler stars. Take a look at this picture to see several bluish stars inside M67, including a very blue star of spectral class B.)

M67 is located far from the plane of the Milky Way, which may be one of the reasons that this open cluster has existed for such a long time. Open clusters usually disperse when they are still quite young. The reason for their rapid "evaporation" is that they are typically relatively low in mass, and also they typically form in the thickly populated plane of the Milky Way, where there are all sorts of gravitational influences that can tear clusters apart.

NGC 7789 appears to be located in the plane of the Milky Way. Clearly it is also a rich and therefore a massive cluster. Its comparatively large mass must be the reason why it has managed to keep itself together and grow so old that it is full of bright red giants.

Ann

I "see" other things as well, but I do see Petal shapes....

Really neat Image...

:---[===] *

Beyond wrote:I dunno. It just doesn't strike me as looking like a rose. But what do i know, I'm a landlubber, an astro-nothing.

I think it probably looks like a rose when you look at it through a telescope. Some of these popular names are from the visual view in a telescope rather than photographic.

Also I think some of the yellow colour is due to dusty molecular clouds between us and the cluster.

starsurfer wrote:

Beyond wrote:
I dunno. It just doesn't strike me as looking like a rose. But what do i know, I'm a landlubber, an astro-nothing.

I think it probably looks like a rose when you look at it through a telescope. Some of these popular names are from the visual view in a telescope rather than photographic.

I too share your views but I trust there must be a record of the picture on which the name was chosen. So here it is. Now it makes sense.


Here is the complete article: http://www.google.ca/imgres?imgurl=http ... CDkQ9QEwBA

Yeah, that'll do it.

Thanks Ann for the interesting comparisons, and Ernie for the sketch of Caroline's rose. That "rose" looks rather suggestive!

NGC 7789 is an interesting and lovely cluster through binoculars or a small telescope, as easy to see in a reasonably dark sky as most of Messier's open clusters, although her edges are rather diffuse, which makes her harder to see through city light pollution. Fortunately she's really easy to find, smack dab between 5th magnitude rho and sigma cassiopeiae. She does look more red than most open clusters to my eye. For northern hemisphere observers, Cassiopeia is high in the northeast after dark and at her highest due north before midnight during autumn, so you might as well go have a look for yourself this evening.

It's great to see Caroline Herschel remembered in the sky.

Thanks, Anthony! I can't resist pointing out yet another cluster, Trumpler 5 in Monoceros. It is located seemingly and perhaps in reality very near the Cone Nebula and the Christmas Tree Cluster (NGC 2264). This Harvard paper estimates the distance to Trumpler 5 to 2.4 +/- 0.5 kpc, and Jim Kaler believes that the distance to S Mon in the Christmas Tree Cluster is 2500 light years.)

Whatever the proximity (or not) between the Cone Nebula and the Christmas Tree Cluster on one hand and Trumpler 5 on the other, it is clear that Trumpler 5 is very dust-reddened and also very old for an open cluster. According to the Harvard paper I linked to above, the age of Trumpler 5 is 5.0 +/- 0.5 Gyr, likely making it older than M67 and much older than Caroline's Rose. But the very red color of Trumpler 5 is more due to dust-reddening than to age, although both factors contribute.

Finally, let me say that I agree with you, Anthony, that it is good to see Caroline Herschel remembered in the sky with a beautiful cluster named after her!

Ann