The Strange Case of BL Lacertae
- Last Updated: 08 October 2014
By David Nakamoto
Serendipity has always been part of the process of discovery. Facts are collected. Perhaps they sit in cabinets and drawers for years, even decades, before someone takes the time to study them thoroughly. And perhaps something unusual is discovered. Even discovering and analyzing a point of light can bring surprises. In this case, it happens to be a ho-hum variable star.
When photography developed to the point where the photographs had value as accurate representations of what was in the sky, they became one of the most valuable resources in the scientific study of the heavens. Now observers had the means to record everything exactly as it is on any particular night. This gave them the ability to compare the sky with what it looked like years, even decades before. This was the technological breakthrough that opened up the possibility of large scale examination of the sky for asteroids, variable stars, and anything else that changed either its position and/or brightness.
Thus, at the beginning of the 1900s, a few observatories were very involved into discovering variable stars via photography. One of these was the Sonneberg Observatory in Germany. Because of the nature of this work, they built up some of the largest collection of photographic plates, spanning both the sky and time.
So Sherman, set the Way-Back machine to 1929.
1929: A Ho-Hum Star
The scene opens at the Sonneberg Observatory. On an examination of plates taken in the constellation of Lacerta the Lizard a new variable was discovered. The 14th magnitude star displayed irregular fluctuations with a range of about 3 magnitudes but no other oddity. However, the irregularity of the star's brightness posed a problem. Periodic variables such as Cepheids and Mira-type variables lend themselves more easily to theoretical predictions and testing, but irregular fluctuations don't, so the new star received no attention from astronomers, and was soon forgotten.
At left is an 18 by 14 arc-minute image courtesy of the STScI Institute. BL Lacertae is dead center in the image.
No wonder it was forgotten.
Now Sherman, set the Way-Back machine to 1968, when things started to happen.
1968: A VERY Strange Star
After being ignored for so long, in 1968 two Canadian astronomers from the University of Illinois, J.M. MacLeon and B.H. Andrew, identified a radio source with an unusual spectrum in the region. Later J.L. Schmitt identified it as BL Lac. Soon more peculiarities came to light. The radio strength would vary in as short a span as one month. And the emission was polarized, with the direction of the polarization varying from week to week.
Then more surprises came from the optical guys. The brightness was found to vary rapidly, often from day to day. Then someone finally got a low dispersion (resolution) spectrum of BL Lac, and uncovered the biggest surprise. The spectrum contained no lines.
No emission lines.
No dark lines.
It was as flat as a sheet of paper. And just as blank.
Now, this last fact was seen only in some unusual white dwarfs. In these cases, the cause is simple. White dwarfs are stars cooling off, with no fusion burning occurring. Their heat comes from the fact that they are the cores of stars that used to produce energy through fusion, and once this stopped the tremendous heat in the core would radiate away, similar to the way heated plates and dishes slowly cool even though you removed them off the burner. Hence they can emit perfect thermal emission or black body radiation, so if nothing is burning on the surface and there is no atmosphere above the dwarf, the spectrum is a continuum with no lines.
But then astronomers did a color index analysis. This involves measuring the brightness in one wavelength in the ultraviolet at 3640Å (Ångstroms or 0.1 nanometers), one in the blue at 4420Å, and one in the visible at 5400Å. When this was done, BL Lac didn't fit into the known sequence of stars. However, it DID fit in with quasars and other unusual galaxies, all with bright stellar cores. This implied that it might be extragalactic. But if it was star-like in appearance that implied great distance, and if it was extragalactic and at the distance of the quasars but if magnitude 14, it had to be one of the most luminous objects known.
Then astronomers found a few more objects with similar characteristics to BL Lac: relatively quick variability in both radio and optical wavelengths along with variable polarization, no lines in the optical spectrum, and a stellar appearance in both radio and optical wavelengths. These objects were called BL Lac after the progenitor of the class.
But why no lines in the spectrum? In 1967 Geoffrey and Margaret Burbidge proposed the properties that might be exhibited by quasars with large blue shifts, and now it appeared that many of these properties were present in BL Lacs. Remember, in the 1970s there was still some debate and controversy about the large red-shifts of quasars, and what caused them.
On the other hand, in 1974 S.T. Shapiro and J.L. Elliot proposed that BL Lacs could be isolated black holes accreting gas and dust. The observed emissions would be coming from the inner part of the accretion disk, where gases would be heated up through friction. This would explain everything; the radio and visual properties, and the spectrum. Therefore the Big Issue was how far BL Lac to us.
But there was just One Problem. Because the spectrum was featureless with no lines, it was impossible to measure any shift in the spectrum, let alone a red-shift. And BL Lac failed every other attempt to measure its distance.
Curiouser and curiouser.
1973: Looking for a Firefly Perched on a Searchlight
Then in 1973, John B. Oke of Caltech and James E. Gunn of Princeton came up with an interesting idea.
They accepted the fact that the spectrum of BL Lac had no lines and therefore couldn't be used to measure its distance.
However, they wondered if BL Lac was just the bright core of a galaxy, and therefore was surrounded by the much fainter body of that galaxy. The problem was the brightness of the core, which could mask the light from the surrounding regions. So they placed a ring diaphragm on the 200-inch Palomar reflector, to cover up BL Lac but allow the surrounding region to be photographed through a spectrograph. When they did this, they obtained a spectrum that showed faint spectral lines, which matched those of a large elliptical galaxy. But those lines also showed a red-shift of z = v/c = 0.07, where v is the object's speed and c is the speed of light. Based on the most recent equations and constants, this means a distance of 900 million light-years.
So BL Lac was definitely extra-galactic . . . right?
Well, soon after Oke and Gunn published their report, J.A. Baldwin of Lick Observatory rained on their parade by using the observatory's 120-inch reflector, and reported that he couldn't see this spectrum, and contested Oke and Gunn's report.
But Oke and Gunn were not discouraged or outdone. Once again they undertook to obtain the spectrum of BL Lac in 1974, but this time they took pains to observe when BL Lac was near the zenith (it does pass almost overhead for people in mid-northern latitudes). Further, they observed on nights when BL Lac was at minimum brightness so the light from the suspected core wouldn't overwhelm the light from the suspected galaxy. When they did this, they saw the previously reported spectral lines . . . quite conspicuously. Furthermore, they matched their previous measurements. So BL Lac was a large elliptical galaxy with an exceptionally bright core.
Twinkle, Twinkle, Little . . . Stars?
In the meantime, two other BL Lacs, the "variable star" AP Lib in Libra, and the "planetary nebula" PKS 0735+178, were also found to have red-shifts of z = 0.049, and z = 0.424 for the latter. The image at left shows AP Lib, courtesy of the STScI institute. In his case, you can identify it because it shows up as a fuzzy image. But as you can see, these objects look like stars, and have been confused in the past as stars along with quasars.
So by the end of the 1970s, many similar objects were identified. But if these objects were extra-galactic and so bright, what exactly were they? What was going on with these galaxies, with these extraordinary bright cores, such bizarre properties, apparently confined to a small space?
This latter property was inferred from the rapidity of the optical light output. Assume the object is of a certain size and it's a sphere, but you only see it as a point of light due to its distance. Let's say that the entire surface of the object brightens at the same time. Take a point at the very front of the sphere, which is also closest to the observer. Call that point A. Now pick a point on the rim as seen by the observer. Call that point B. Now, if the entire sphere brightens at the same time, the light from point A is closer to the observer and will reach that observer sooner than B. Hence the object seems to brighten gradually, and the speed of the brightening is directly related to its size. If the object brightens in one month, then the radius of the sphere is one light-month.
But BL Lac objects can change their light output in days, sometimes hours. This implies that the light we see comes from a source only light-hours across. This might be large compared to the Solar System which is six light-hours in radius, but since the distance to the nearest star is over 4 light-YEARS . . .
And the cores of galaxies are even larger. So if BL Lac is the core of a giant elliptical galaxy, this core, or the active region, must be very small compared to most galaxies.
But now BL Lacs were connected, at least by distance, to quasars, so their explanation was tied to those objects. And on that note, my tale comes to an end.
Not bad for a long forgotten star.
Take a Look
You can try and see BL Lac. The coordinates are: RA = 22h 02m 43.3s, Dec = +42° 16′ 40″. It should be positioned well during the late summer months into the fall, but trying to separate it from the other stars in the crowded field is another matter, even for those using imagers.
AP Lib is of course in Libra, further south than BL Lac. Its coordinates are: RA = 15h 17m 41.8s, Dec = -24° 22′ 20″
Happy Observing !