April 07, 2008


Today, I will continue my efforts to fill out the Southern part of the constellation map with another faint, small constellation that was invented by Hevelius. The constellation Sextans fills in a small area of the sky directly south of Leo. In fact, the brightest star in Sextans, nearly 5th magnitude (so invisible without very good viewing conditions) is 12 degrees due South of Alpha Leonis, the bright star Regulus. There are no truly remarkable stars in this constellation, so I'll skip directly to the deep sky objects.

Probably the most well-known is NGC 3115, a lenticular galaxy sometimes known as the Spindle Galaxy. This is a spiral galaxy without the characteristic spiral arms and without much star formation as it appears the gas and dust in the disk has been pretty much used up. This can be found another 7 degrees south of Alpha Sextantis, or 19 degrees due South from Regulus. Closer to Leo, if you follow a line from Regulus to Rho Leonis and again as far in the same direction (maybe a little to the South of this line) is the face-on spiral galaxy NGC 3423, about 37 million light years from our own Milky Way. Lots of obvious star-forming regions here with their signature H-alpha emission.

Somewhat West of this is a very nice galaxy pair, NGC 3166 and NGC 3169, found just a half degree east from a line connecting Regulus and Alpha Sextantis, about 3/4 of the way from Regulus. These two interacting spiral galaxies are a mere 50,000 light years apart and about 60 million light years distant, so a tough target for a small telescope. Notice how blue 3169 is on the upper right of the linked photo, indicating there is still plenty of ongoing star formation compared to its compansion.

Another very nice face-on spiral in this part of the sky is NGC 2967, found 11 degrees due West from Alpha Sextantis, on the border with the constellation Hydra. This very faint (80 million light years distant) galaxy is reminiscent of the Whirlpool galaxy, M101, and it has a very large angular size on the sky. The light from its edges fade so gradually, it is very difficult to tell where the true edge of the galaxy is located.

The final deep sky object is in the center of the constellation about 4 degrees due South of Alpha Sextantis: the dwarf galaxy Sextans A. This is a tiny irregular galaxy and a member of our Local Group of galaxies, only 10 million light years away and a square shape about 5000 light years across. In the linked image, you can see that even though this galaxy is small and faint, there are at least three clear regions of ongoing star formation.

Posted by Observer at April 7, 2008 09:35 PM

Comments on entries can only be made in pop-up windows while those entries are still on the main index page. Sorry for the inconvenience this causes, but this blocks about 99.99% of the spam the blog receives.

Astro-geeks have long asserted that we need to go on a crash program of a deep search for variable stars in Sextans in order to discover and locate EZ Sex.

A viable topic for one of your essays is explain what all that means.

Posted by: Feff on April 9, 2008 09:13 AM