Constellation of the Month: Lacerta, the Lizard

Johannes Hevelius is at it again with this month's constellation, Lacerta, the Lizard. Lacerta is a small constellation between Andromeda and Cygnus. This area had previously been named Sceptrum et Manus Iustitiae (in English, Sceptre and the Hand of Justice), Stellio the Newt, and Frederick's Honors. As with so many of Hevelius' constellations, its brightest star is a faint magnitude 3.8. Unlike many of Hevelius' constellations, this one is in the Milky Way and has a number of interesting objects.

Click the image for a larger map Our constellation of the month, Lacerta, the Lizard, is two-thirds of the way up in the northern sky, east (right) of the meridian. This map is how the sky looks about 7:30 p.m. when facing north and looking up about 60 degrees above the horizon. Lacerta is surrounded by Cygnus, Pegasus, Cepheus and Andromeda. The stars that make up Lacerta are fourth magnitude and fainter, so you will have to look carefully to find it in our October sky.

One object, a supergiant star, 4 Lacertae, is over 2,000 light-years away, but is still magnitude 4.55. Compare that to Alpha Lacertae, which is only 102 light-years away, 20 times closer, but only twice as bright. Since brightness is related to the square of distance, 4 Lacertae must be some 200 times brighter than Alpha Lacertae. How was that calculated? Take the 20 times distance difference and square it; you get 400, which is how many times brighter 4 Lacertae would have to be if it were to be the same magnitude as Alpha Lacertae in our sky. Since 4 Lacertae appears only half as bright, it must be half of 400, or 200 times brighter than Alpha. This is why 4 Lacertae is called a supergiant.

One of the most interesting objects appears to be a 14th magnitude star called BL Lacertae. BL was originally believed to be a variable star when it was discovered in 1929. Its brightness changes without any pattern in periods as short as one day. When sensitive radio telescopes were built in the 1970s, they discovered that BL Lacertae was a strong radio source. This cast doubt on the identification of this star-like object as a variable star, which could not emit such prodigious amounts of radio waves.

BL Lacertae, and other objects like it, had no emission lines, the markers that allow astronomers to determine the "red shift" and from that the distance to these objects. Deep optical observations of this object in 1974 showed a faint fuzziness surrounding BL Lacertae, proving that it is actually a very distant galaxy with an extremely bright core. The bright core is called an "active galactic nucleus" (AGN), and the whole object is called a "blazar," a term coined in 1978 by astronomer Ed Spiegel.

Imagine if the center of our galaxy in Sagittarius glowed hundreds of times brighter than the full moon. This would be an amazing sight in our sky, but that is exactly what is going on at the center of these distant galaxies.

At the heart of BL Lacertae, and all blazars, is a supermassive black hole. This tiny object, barely 10 light-years across, has the mass of over a hundred of our suns. The huge mass of the black hole pulls material from the surrounding galaxy toward it. This material forms a disc around the black hole controlled by the hole's magnetic field.

As gas and dust pile up in the disc, some of the material nearest the black hole is pushed inward from the accretion disc toward the black hole. This material is torn apart and some of the resulting electrically charged particles are pulled along the magnetic field that comes out of the north and south magnetic poles of the black hole. These particles form two jets shooting in opposite directions out of the black hole. In a blazar like BL Lacertae, one of these jets is aimed almost straight at us, like shining a flashlight in our direction. This gives us the appearance of a very bright nucleus and the rest of the galaxy being almost invisible by comparison.

The Planets for October 2007

Still hanging on in the western sky as it gets dark, Jupiter is moving slowly eastward among the stars of Ophiuchus. At magnitude -2.0, Jupiter sets about 9:30 p.m. As it gets dark, Jupiter is lower in the southwestern sky than it was last month, making it no longer a prime target for observing. At mid-month, Jupiter is 34.0 seconds-of-arc across.

Mars is next up in our evening sky. Rising around 11 p.m., Mars is 10.8 seconds-of-arc across and growing as the Earth gets closer to it. The God of War is magnitude -0.3 and is moving eastward in Gemini all month. It is more than large enough to warrant telescopic observation as we head toward opposition in December.

Venus and Saturn are both spending the month in Leo. At the beginning of October, Venus comes up first around 3:45 a.m., followed 30 minutes later by Saturn. In between is the first-magnitude star Regulus. By the end of the month, Regulus will come up first, followed by Saturn at 2:30 a.m. and finally Venus about an hour later. At mid-month, Venus is 28.1 seconds-of-arc across its 43-percent sunlit crescent and shines at magnitude -4.5. At the same time, Saturn is 16.7 seconds-of-arc across while the rings are 37.9 seconds-of-arc across, tilted up 8.0 degrees to our line of sight with the southern face showing. That morning, Oct. 15, Venus passes 3 degrees south of Saturn, so take a look at the two planets together in Leo and "keep watching the sky"!

An amateur astronomer for more than 35 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.

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