June 2011 Sky from the Keeble Observatory
Last month, we were going to see how the Type Ia supernovae provide us a “standard
candle” for measuring the size of the Universe. However, the realities of end-of-semester
and final projects and final exams meant that the intended May posting never happened.
Let’s recall some key points in the evolution of “low mass” stars like our Sun.
These spend billions of years as main sequence stars, fueled by the fusion
of hydrogen into helium at their cores. In the case of the Sun, after 10 billion
years the hydrogen at the core is exhausted. Although hydrogen continues to fuse
in a shell around the core, the helium core cannot fuse until the temperature rises
substantially. This happens after the “shell source” has added enough additional
helium. The star at this time is a red giant, swollen in size to engulf the orbits
of its inner planets, if any. (We’ll be toast by then!) Helium fusion at the newly
ignited core makes carbon (mostly) and oxygen, but this is the end of the line for
energy production. The star will eventually eject its hydrogen envelope, leaving
behind a remnant ball of degenerate carbon – a white dwarf – with almost the mass
of the Sun but compressed into a sphere the size of the Earth.
In the 1920s, Subramanian Chandrasekhar showed that there is an upper mass limit
for a white dwarf. Beyond 1.4 solar masses, the white dwarf will collapse and explode
as a Type Ia supernova. So, where do we get this extra mass to turn 1 solar mass
into 1.4? Recall that most stars are not single, but exist in binary or even triplet
systems. Remember, also, that more massive stars evolve faster. Here, then, is the
The more massive star in a binary completes its main sequence and red giant phases,
leaving behind a white dwarf. Its companion eventually swells into a red giant,
and the white dwarf captures matter from the extended envelope of that star. If
enough is transferred to exceed the Chandrasekhar limit … BOOM! The explosion will
outshine an entire galaxy, even exceeding the luminosity of the massive star detonations
known as Type II supernovae.
It is their brightness and the precision of the Chandrasekhar limit that makes Type
Ia supernovae such excellent standard candles. Since the mechanism is the same,
they all have the essentially the same peak brightness. Measure the apparent brightness
and compare with that peak. The comparison tells you how far away the explosion
Lunar phases for June: New Moon on the 1st, at 5:03 pm; First
Quarter on the 8th, at 10:11 pm; Full Moon on the 15th at
4:14 pm; Last Quarter on the 23rd, at 7:48 am.
Pre-dawn planet watchers can concentrate on the eastern horizon. Early in June you
will see Jupiter rise about 90 minutes before sunrise, followed an hour later by
Venus and Mars – though the angle of rise will be shallow, and you may not be able
to see them in the horizon clutter and glare from the Sun. By the end of the month
you’ll have an earlier shot at Jupiter, rising 3 hours earlier than the Sun. Mars
will be an hour ahead of the Sun. Venus and Mercury will be too low to pick out
of the glare.
Evening planet watchers will have to content themselves with Saturn, which emerges
from twilight high to the south. It sets about six hours later. By the end of June,
look for it a bit further west, and setting about five hours after sunset.
Looking straight overhead at mid-month, about two hours after sunset, you will find
the constellation Bootes (the Herdsman). Its brightest star is Arcturus, about 20
degrees southwest of zenith. To the northwest, handle high and almost vertical,
you’ll see the familiar shape of the “Big Dipper” in Ursa Major. To the west, look
for the familiar “sickle” of Leo. Its brightest star, the heart of the Lion, is
Regulus, about 20 degrees above the horizon. Virgo lies to the southwest. Spica
is its brightest star, about 20 degrees left of Saturn. Low to the south you will
see Scorpio, with the bright red Antares about 20 degrees off the horizon. East
and northeast, look for the Summer Triangle of three bright and widely spaced stars
– Deneb, Vega, and Altair. Cygnus, the Swan is to the northeast, with Deneb at about
30 degrees altitude. To the east, bright Vega marks Lyra, about 50 degrees from
the horizon, and Altair in Aquila is below at about 20 degrees. The Summer Triangle
will climb higher as we move through the long, hot summer.