January 2011 Sky from the Keeble Observatory
In last month’s column, we traced the evolutionary track of a star like the Sun.
One solar mass of hydrogen and helium (roughly 75% H, 25% He by mass) collapses
to form a star, powered by nuclear fusion at its core. It grows brighter and hotter
over time, reaching the observed current luminosity and surface temperature after
about 5 billion years, about half-way through its main sequence lifetime. In another
5 billion years, the Sun will exhaust its core hydrogen and leave the main sequence,
swelling into a red giant. The radius at the photosphere (the visible surface) will
be somewhere near the current orbit of Mars. The bright star Betelgeuse, in the
constellation Orion, is an example of a red giant.
This expansion is powered, surprisingly, by an increased rate of fusion. The core,
which is now mostly helium, contracts and actually gets hotter due to the liberation
of gravitational potential energy. This triggers fusion in a thin shell around the
core. The fusion rate is proportional to the fourth power of temperature, so the
shell luminosity vastly exceed that previously provided by the core. Some of this
increased power pushes the envelope of the star outward; the rest increases the
surface luminosity, albeit at a lower temperature. (Luminosity is proportional to
both surface area and the fourth power of temperature. Even at the lower temperature
… hence the reddish appearance … the surface area is vastly greater.)
The red giant phase for a one solar mass star will last about a billion years. The
helium core will grow larger as the shell fusion produces more helium. Eventually
this core will be hot enough to trigger fusion of helium to carbon. This marks the
beginning of the end for our Sun. Within a few million years the outer envelope
of the star, tenuously held by gravity, which falls off inversely with the square
of distance from the center, will be ejected into space. The expanding shell of
gas is known as a planetary nebula, like the Ring Nebula in the constellation Lyra.
(Google “planetary nebula” and you can find lots of images, many of them strikingly
beautiful.) The remaining core is what is known as a white dwarf. Roughly the size
of Earth, with a surface temperature in excess of 40,000 K, this remnant has no
fuel source since it’s not massive enough to trigger carbon fusion. Rather, it will
continue to cool slowly over the following eons.
Stars less massive than the Sun will follow a similar track, but take much longer
to do so. Further, below about half a solar mass, the star will never initiate helium
fusion, so the remnant white dwarf will be helium rich rather than carbon.
Next month, we’ll look at the tracks for stars more massive than the Sun.
Lunar phases for January: New Moon on the 4th, at 4:03
am; First Quarter on the 12th, at 6:31 am; Full Moon on the 19th
at 4:21 pm; Last Quarter on the 26th, at 7:57 am.
Predawn planet watchers will find Venus as the brightest object to the southeast.
Look for a brilliant “star” about 25 degrees above the horizon … you can’t miss
it. Saturn begins the month high to the south, about 45 degrees above the horizon.
It will rise earlier each morning, and you’ll see it to the south-southwest by month’s
end. Evening planet watchers will have to content themselves with Jupiter, which
begins the month emerging from twilight about 45 degrees above the south-west horizon.
Later in the month it drifts lower and closer to the western horizon.
At mid-month, about two hours after sunset, we find the constellation Perseus at
zenith. This is not home to a lot of bright stars, but you can find the interesting
variable binary Algol about 8 degrees to the northwest. (My undergraduate advisor
discovered a third component to this system, and elicited groans from a meeting
when he announced that “Algol is divided into three parts.” Students of Latin and
readers of Caesar will know whence that comes!)
Turning to the northeast, bright Capella, about 58 degrees above the horizon, is
the brightest star in the constellation Auriga. Behind you, to the southeast at
65 degrees elevation, Aldebaran marks the bright heart of the Bull, the constellation
Taurus. The Pleiades are to the south, a bit higher at 76 degrees. To the east,
we see the bright “twins” of Castor above Pollux, in the constellation Gemini. They’re
not even the same brightness, but they’ve been paired as such since antiquity. Below
these two, at about 23 degrees your binoculars will reveal the open cluster known
as the Beehive.
Orion needs no introduction!