September 2010 Sky from the Keeble Observatory
Our previous column discussed how we determine the masses of stars, and that we
now know that main sequence stars of the same mass are also stars of the same effective
temperature and luminosity. That is, a five billion year old star of one solar mass
is going to be almost identical to our Sun.
As we noted several months back, one of the standard tools of any scientist is to
look for correlations among the various properties observed for whatever is under
study. In the case of main sequence stars one of the striking correlations is between
mass and luminosity. Over the entire main sequence we find that more massive stars
are also hotter and more luminous, with luminosity increasing roughly as the cube
of the mass. This “mass-luminosity relation” is one of the most important things
we can know about stars. Why? Luminosity essentially tells us the rate at which
the star is using its “fuel” – and mass tells us roughly how much fuel is available.
Think fuel economy ratings for cars as an analogy. If your car gets good mileage,
you can travel farther and for a longer time on a tank of gas, even if the tank
itself is smaller. If we divide the amount of fuel (the mass) by the rate at which
fuel is used (the luminosity) we have the lifetime of the star, and this works out
as inversely proportional to the square of the mass.
What is this telling us? Consider a star of one solar mass, essentially identical
to the Sun. We know that the Sun is about 5 billion years old. Our computational
models tell us that the Sun will exhaust its core hydrogen in another 5 billion
years, which will cause it to evolve into a red giant (more about this next month).
Thus, a star of one solar mass has a main sequence lifetime of about 10 billion
years. A star of two solar masses will stay on the main sequence for only about
2.5 billion years – ten billion years divided by 2 squared – but will be 8 times
brighter than the Sun. For 10 solar masses the lifetime is only 100 million years,
but the luminosity is a thousand times greater. For the lowest mass stars, about
a tenth of a solar mass, the lifetime becomes an incredible ten trillion years –
well beyond the current age of the universe itself. Why is this important? It means
that if you see a massive star in the sky, it has to be young! In other words, stars
are still being formed … the universe is dynamic and evolving, rather than simply
being static and with all the stars and galaxies made at the same instant.
Lunar phases for September: Last Quarter on the 1st, at 1:22 pm;
New Moon on the 8th, at 6:30 am; First Quarter on the 15th,
at 2:14 pm; Full Moon on the 23rd at 5:17 am. And, there will be a second
occurrence of Last Quarter on the 30th at 11:52 pm. One time zone further
to the east, and this would be happening on the 1st of October!
The autumnal equinox will take place at 11:09 pm on the 22nd. At that
time the Sun will cross the celestial equator moving south. Daytime and night time
will be almost exactly 12 hours. This month’s New Moon takes place only a day after
perigee – its closest approach to Earth – so expect higher tides than usual.
Predawn planet watchers will find Jupiter to the west southwest, about 20 degrees
above the horizon, early in September. With binoculars you may also be able to discern
Uranus about one degree to the west. Jupiter reaches opposition on the 21st,
by which time it will set as the Sun rises. Later in the month you will see Mercury
returning to the early morning sky, rising to the east about an hour before sunrise.
Early evening skies will be graced with a nice display to the west southwest after
sunset. Venus and Mars are both near the bright star Spica, about 17 degrees off
the horizon, and Saturn is about 20 degrees further west and just a little lower.
Jupiter rises earlier and earlier to the east. By the end of September Venus and
Mars will move lower to the southwest and Saturn will be lost in the Sun’s glare,
reaching superior conjunction on the 30th. (Planets farther from the
Sun than we are can be at opposition, i.e. 180 degrees around the ecliptic from
the Sun, or at superior conjunction, on the other side of the Sun. Planets closer
to the Sun, i.e. Mercury and Venus, can never be at opposition, but can be at either
inferior or superior conjunction.)
Our mid-month view of the sky, about two hours after sunset, finds Cygnus (the Swan)
at zenith, appearing to glide along the Milky Way from northeast to southwest. The
bright star Deneb marks the tail of the swan, about ten degrees northeast of zenith.
At a similar angle to the west we see brilliant Vega in the constellation Lyra.
These two bright stars make a triangle with Altair, about 30 degrees south of zenith.
At this hour, Jupiter is to the east southeast, midway between Pisces on the eastern
horizon and Aquarius to the southeast. Following the ecliptic through the constellations
of the zodiac we will see the Moon in Sagittarius on the 15th, about
25 degrees above the south southwest horizon, and Antares in Scorpio to the southwest.
Cassiopeia is to the northeast, looking like a W tipped backwards. If you follow
the line of the “left” side of the W toward the east you’ll notice the faint fuzzy
patch of the Andromeda Galaxy, which is the most distant object visible to the naked
eye. This spiral galaxy is only 2.2 million light years away, and is on a collision
course toward our home Galaxy … it will be “here” in about 5 billion years, roughly
when the Sun becomes a red giant!