Can You Find a Quasar? (You Might be Surprised)
May 2010 :
I’m sure everyone who attended the March meeting of Skyscrapers enjoyed Prof. Savvas Koushiappas’ interesting talk on extreme astrophysical objects viewable to amateur astronomers. I was reminded of two articles I’d done on such objects which were in the February and September 2009 issues of this newsletter, which detailed how to find the most easily seen white dwarf star (February 2009) and the visible companion star to the black hole “candidate” Cygnus X-1 (September 2009). I thought readers having 8” or larger telescopes might wish to observe a quasar, provided you can follow a detailed finder chart and manage to isolate a 12.8 magnitude object that looks exactly like a star, but actually is not. Rather it’s the brightest known quasar and, if you can successfully locate it, will be by far and away the most distant thing in the universe most of you will ever see in your lives. (Assuming that the consensus of most cosmologists in assigning a theorized distance of about 2.5 billion light years is close to the truth–there are competing ideas as to exactly what quasars are, and how far away they might really be.)
You can easily find information on the history of observation, research, and identification of “quasi-stellar objects” that are termed quasars, so I’ll avoid taking up much space here on the subject. Briefly, quasars are thought to be the superluminous cores of certain “active” galaxies, capable of radiating over a trillion times as much energy as our Sun from a compact region not much larger than our Solar System. Back in 1963, Dutch-American astronomer Maarten Schmidt became the first person to develop a satisfactory explanation for the extremely puzzling spectrum of the starlike object 3C?273 in Virgo. Schmidt figured that unusually broad emission lines in the spectrum were actually quite familiar hydrogen lines shifted in wavelength to a degree not realized prior to his work. The enormous red-shift implied a staggering cosmological distance of at least a billion light years for an object that had no apparent angular size as viewed even in the Hale Telescope at Mt. Palomar. How could a single star possibly be so extraordinarily brilliant so as to be detected visually at such a fantastic distance? Such a compact source, Schmidt conjectured, would need to be as luminous as a hundred entire galaxies! (I’ll leave the rest of the story to those inclined to read up on this matter–you’ll find many references online and in various astronomy books.)
I’m going to describe a star-hop that will get you very close to 3C?273, but you’ll definitely need a good finder chart to positively zero in on this quasar. Remember: You should observe under reasonably dark, transparent skies with minimal local light pollution and have at least an 8” scope; 10” is better still. Late at night in mid-May is amenable for this particular search. The Moon is New on May 15th, which will be of great help. There’s a neat website called Washed-out Astronomy (http://washedoutastronomy.com/content/3c-273-quasars-are-easy) that might really help you. Once you go to the site, check near the bottom for easy links to five different AAVSO finder charts you can call up. (The “A” chart is a 15° field of view; the “E” chart goes down to 0.5° field of view.) Also, the chapter on Virgo in Volume 3 of Burnham’s Celestial Handbook features both a good finder chart on page 2101 and a finder photograph on page 2105. I’ve mentioned this book because many amateurs own it or can somehow access the book, which is a marvelous guide to astronomy even though it contains certain information current only to 1978.
Start a star-hop by picking out Spica (Alpha Virginis), the 15th or 16th brightest star in the entire sky. Both Spica and Antares in Scorpius are very slightly variable, so the exact brightness ranking of Spica at any given time is dependent on its precise magnitude compared to that of Antares. Spica averages about 0.98 magnitude and serves as the benchmark for 1st magnitude stars. From Spica, go about 14.5° northwest to the famous (and difficult) double star Gamma Virginis, Porrima, which marks the basic center of the star pattern outlining Virgo’s form. Gamma Virginis has a combined magnitude of 2.8; each component is about 3.5 and the two stars currently are perhaps 1.4 arcseconds apart, but widening somewhat rapidly. This ranks among the very best double stars in the sky. The members of this system are F0 class stars; at 0.5° to the east-southeast of Gamma will be seen a 5.9 magnitude star of class G8. Gamma’s widest separation, by the way, will reach about 6 arcseconds around the year 2080.
Virgo is the 2nd largest of the 88 constellations and has 9 stars brighter than magnitude 4.0, but it contains only 3 brighter than 3.0: Spica, Gamma (Porrima), and Epsilon (Vindemiatrix, at magnitude 2.85). The following table neatly organizes some important basic information for 8 objects (including Saturn, as of May 15, 2010) involved in star-hopping to 3C?273. As I mention each star in the text, you can note its coordinates of right ascension and declination (as per the year 2000) so as to more easily locate it on a star atlas. I’ve also included each star’s apparent (visual) magnitude and its spectral class, which will indicate the predominant color. This should make the text a bit neater, less wordy, and more straightforward to follow. From Gamma, go 5.5° west-northwest to 15/Eta Vir, also known as Zaniah. This star is naked-eye visible on clear, dark nights and lies about midway between Saturn (in May of this year) and Gamma Vir, although a bit S of a line joining these objects. You’ll easily verify Eta in a scope because it has a very wide optical companion about 0.3° to the WSW, magnitude 5.9 13 Vir, class A5. Eta Vir is the last truly naked-eye star involved in this star-hop, although the next star we’ll use might be, depending on circumstances and your own vision. Go 4.0° due north of Eta to 16 Vir, which is marked as a double on some atlases. (The companion is too far from the primary and much too faint to be of any interest.) The north-south line joining Eta and 16 is critically important here, because it represents one side of a nearly equilateral triangle that will allow you to find the next star necessary for this search, a wide and easy double of 50 arcseconds in separation: A?2583, also known as ADS 8582. From the midpoint on the line joining Eta and 16 Vir, go 2.8° due E to find A?2583 and you’ll be only about 0.9° away from 3C?273. A?2583 is quite prominent; no other double star comparable to it lies anywhere close-by. Remember, lines running to this star from both 16 Vir and Eta Vir form 2 sides of a triangle pointing due east. Although not precisely equilateral, it’ll be close enough.
While looking in this area, by all means stick with the lowest power, widest field eyepiece you’ve got, particularly if your scope is of a focal length long enough to make really low magnification impossible. Consider about 50x as the approximate high limit for magnification for this star-hop, and 30x to 40x is better still. I assume that some kind of light pollution or sky-contrast filter would enhance faint star visibility, especially in larger scopes of 12” aperture and up. Once you’ve centered on the region of 3C?273, you’ll want to increase magnification (if possible) so as to darken the sky background a little and be able to make out fainter stars. If possible, don’t miss the fine red carbon star SS Vir, roughly midway from A?2583 back to Eta Vir. This easy star is actually just a bit north of the line joining those two stars, which is one side of the triangle I’ve described. SS Vir stays fairly bright throughout its entire period of 355 days, as you’ll see in my table.
One other star nearly midway along a second line running from A?2583 NW to 16 Vir is important, because it may be used as a reference point in conjunction with A?2583 so as to isolate the region of 3C?273. The star is HD 108228 and is easily the brightest star anywhere close to the midpoint of the line just described, but lies a bit south off the line itself. HD 108228 is 1.5° to the west-northwest of A?2583. You can imagine the 1.5° line joining these two stars as being the long side of a semi-flattened, not quite symmetrical “coathanger” triangle, with 3C?273 located near the hook of this little “coathanger” at a point north of the 1.5° line forming the bottom. If you can make it this far, you’re ready for a finder chart to take you the rest of the way–it’ll be essential. A good tip I can give you is to use the two stars of the double A?2583 as pointers; just extend a line starting from the brighter member through and beyond the fainter companion roughly 0.6° northwest to the first small asterism you’ll encounter, a short, curved row of three stars having the brightest one at the southwest end. Use this row to track to the north-northeast to the first semi-bright star you’ll see; it is magnitude 10.3 and is key to finally locating the magnitude 12.8 quasar.
This magnitude 10.3 star lies at the east-southeast tip of an elongated, nearly right triangle having an 11.9 magnitude star at its southwest corner, at one end of a short line marking the base. A 12.6 magnitude star marks the other corner of the triangle’s base, at the north corner. One of the two long sides of the triangle runs from the 12.6 magnitude star southeast to the 10.3 magnitude “tip.“ 3C?273 is the brighter, slightly more bluish member of what’ll look like a wide double star along this line–it is due east (by 3 minutes of arc) of its 13.5 magnitude “companion” and lies closer to the bright, 10.3 magnitude “tip” of the triangle than that 13.5 magnitude star I’ve described. Only one star will be noted midway along the line marking the other long side of the triangle. It is 12.7 magnitude and lies a bit nearer to the “tip” star than to the other end of this line, denoted by the 11.9 magnitude star. Finder charts showing 3-digit magnitude values with the decimal point omitted are arranged so as not to confuse you; a decimal point could foul things up by being mistaken for a faint star, thus 127 printed next to a star actually means 12.7 magnitude. By the way, another fine finder chart showing this little triangular asterism can be found online at http://www.lsw.uni-heidelberg.de/projects/extragalactic/charts/1226+023.html.
Here’s how the designation 3C?273 is derived: The “3C” means the revised Third Cambridge (England) Radio Survey (catalog); “273” indicates that this object is the 273rd in order of increasing right ascension listed in the survey. The first batch of quasars found were strong radio emitters, but the overwhelming majority catalogued to date are relatively radio-quiet.
There’s no question that locating such a faint, starlike object amid a number of other similarly faint stars is a challenge for many amateurs, but I really think the prize here is well worth the hunt. Your enjoyment comes not so much from seeing a 12.8 magnitude object, but from realizing what it actually is, and how unfathomably far away it lies. To see light this ancient is to look back in time at least half the age of the Earth itself! In closing, you might want to take another look at the May 2009 issue of this newsletter. I wrote a handy guide for finding the best galaxies in late spring (many are in Virgo), entitled “A Better Galaxy Guide, Part 2.”
When to Observe
Constellations
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