Stellar Explosions

The Binary Star SS Aurigae

During many years working in Winfree Observatory at Randolph Macon Woman's College (now Randolph College) I made a number of astronomical observations not widely shared. I'll display a few of these here from time to time. Most of these astronomical events took place over the course of several hours, days, or years, and would escape those with short attention spans.

Winfree Observatory at Randolph College

In the constellation Auriga, about 650 light years away, two stars rapidly orbit each other, completing one orbit in about 4 hours and 23 minutes. The pair of stars is named SS Aurigae (meaning: the variable star system SS located in the constellation Auriga). The position of SS Aurigae is marked with a red X left of the star Menkalinan in the constellation Auriga below.

Location of SS Aurigae (click to enlarge)

One of the two orbiting stars is a red dwarf star whose mass is approximately 40 percent of our Sun's mass. The other star is a white dwarf 10 percent more massive than our Sun. If the orbits were circular, and if we viewed them from above the orbital plane, the orbital motion might look something like this:

The cross marks the center of mass

In the animation above the bigger diameter white circle represents the star with larger mass. This is misleading because the larger mass star in SS Aurigae actually has the smallest diameter. Both stars are smaller than our Sun. The red dwarf's diameter is 47 percent of our Sun's diameter. The white dwarf, however, is much smaller. Its diameter is only 0.3 percent of our Sun's diameter! This means slightly more than a solar mass of white dwarf mass is contained within a volume smaller than Earth! Conclusion: the white dwarf must be tremendously dense!

In order to complete one orbit in 4 hours and 23 minutes the stars must be very close together. Their separation is only about 0.7 percent of the distance between the Sun and Earth! That's only 2.8 Earth-Moon distances apart! These stars orbit so closely the greater gravity of the more massive white dwarf pulls material away from the surface of the red dwarf. This material then spirals toward the white dwarf and forms a disk around the white dwarf before eventually falling onto the white dwarf's surface.

I was lucky enough to observe some of the stellar explosions produced by this system between the years 2001 and 2007 at Winfree Observatory. Here's a graphical display of my observations:

The vertical scale is an astronomer's (logarithmic) scale of brightness measured through a photometric green filter (also known as a visual, or V, filter). The smaller the magnitude, the brighter the star. The horizontal scale shows elapsed time, measured in days, running from July, 2001 to July, 2007. More than ten stellar explosions are shown on the graph above. Here's what one typical explosion looks like on images captured by Winfree Observatory's 14-inch telescope:

Arrows locate the stellar explosion in SS Aurigae (click to enlarge)

In the images above SS Aurigae appears as a single dot. The two orbiting stars are so close to one another, and so distant from Earth, they appear as a single star to us. Even at its brightest SS Aurigae is invisible to the naked eye. It can only be seen with telescopic aid.

Here's the explosion again showing the brightness change that occurred in 19 days from April 15th to May 4th, 2004:

The red arrow points to SS Aurigae

Many stars throughout the image above seem to change shape by small amounts. This is caused by a slight focus difference from one night to another, not because the stars themselves changed brightness. In contrast, SS Aurigae changes from a very small dot to a much larger dot indicating its true brightening.

The animation above may not seem like much of an explosion. We experience no sound, blasting wind, shaking ground, or burning radiation. That's because the explosion is 650 light years away through the vacuum of space! If we were closer it might look like this: