July 2013 Sky from the Keeble Observatory
When I was growing up it was common knowledge that there were nine planets in our
solar system, and there was serious debate about whether there were any more planets
orbiting other stars. What was the debate, and why such a debate? The discussion
went something like this: If the process which forms planets is unique to the history
of our solar system, then such planetary systems should be rare, perhaps limited
to stars like the Sun. If, however, planet formation could be shown to be a byproduct
of star formation itself we would expect planetary systems to be common. The present
consensus among astronomers is for the second possibility, which is confirmed by
the confirmation of nearly 400 “exoplanets” and the preliminary discovery of nearly
500 more. The Galaxy should be the home for billions of planets.
Stars are born from the collapse of cold dense clouds of gas and dust. The clouds
themselves are called “molecular clouds” because about 75% of their mass is in molecular
hydrogen. The dust shields the cloud cores from ultraviolet light, and another molecule
(carbon monoxide) provides an efficient cooling mechanism to keep the cloud cores
only a few tens of degrees above absolute zero. As they collapse under their own
weight (cold gas exerts little pressure, so gravity overcomes the tendency of a
gas to expand) the clouds fragment into smaller pieces which eventually become the
stars. As with every process in nature, there is waste in the sense that only about
99% of the cloud becomes stars. It is the leftover 1% that can become planets.
Close to the Sun the planets that formed are rocky, because volatile materials (easy
to melt and evaporate) like ices cannot condense out of the proto-solar disk. Farther
out we get low enough temperatures so that we get a mix of ices and refractory grains,
and the planets that form are essentially gas and ice with smallish rocky cores.
Between the orbits of Mars and Jupiter we find a swarm of leftover bits that never
coalesced into a planet. These are called asteroids, and we note that the inner
part of this regions contains mostly rocky asteroids, while farther from the Sun
the asteroids are predominantly icy. We find evidence that this process is ongoing
in star forming regions, and we now observe directly the proto-stellar disks and
even evidence for planet formation as an ongoing process.
Oh, and we’re down to eight planets. No, Pluto didn’t go away, but it’s now considered
as a minor planet since it’s not alone. There are, perhaps, thousands of Pluto-like
objects beyond the orbit of Neptune, and Pluto isn’t even the largest one. There
is also one minor planet among the asteroids, known as Ceres.
Lunar phases for July: New Moon on the 8th, at 3:14 am; First
Quarter on the 15th, at 11:18 pm; Full Moon on the 22nd, at
2:15 pm, and Last Quarter on the 29th, at 1:43 pm.
Predawn planet watchers may want to stay in bed early this month. Mars will be low
to the east, but it will fade into morning twilight as the Sun rises. By month’s
end it will be higher, and will be joined mid-July by Jupiter and Mercury. Jupiter
will be the highest and brightest of the trio. Evening viewers will find Saturn
emerging from twilight to the southwest, then setting about four hours after sunset
early in the month. It moves lower as the weeks pass, setting only two hours after
sunset by the end of July.
Our overhead view at midmonth, about 3 hours after sunset (that’s a little after
midnight by the clock), finds bright Vega near zenith in the constellation Lyra.
Recall last month’s note about the “summer triangle” of Vega, Deneb, and Altair.
Deneb is the bright star at the “tail” of Cygnus, the Swan. Look for it about 60
degrees and to the east-northeast. Altair is at about 50 degrees to the southeast.
Following the familiar t shape of Cygnus from Deneb, your binoculars will find Albireo
at the head of the swan. It’s not a terribly bright star, but it lacks any bright
neighbors so it’s easy to find – it lies about 70 degrees above the east-southeast
at this time. A small telescope reveals Albireo as a close binary, with the two
stars of very different colors – one reddish, the other bright blue. The redder
star is cooler in temperature.
Ursa Major is to the northwest, with the bowl of the “Big Dipper” oriented to hold
water. This asterism is also known in some cultures as “the Plow” – the reason is
obvious in this orientation. Turning to the south we find the “teapot” asterism
of the constellation Sagittarius, which marks the direction toward the center of
our Milky Way galaxy. Above and a bit to the right of Sagittarius, don’t mistake
bright red Antares for Mars. The name itself means “against Mars” – though I doubt
there’s any real rivalry. Mars is a ball of rock in our solar system, the 4th
planet from the Sun. Antares is a “class M supergiant” star about 600 light years
from the Sun and about 800 times the Sun’s diameter. If the Sun were placed at the
center of this star, Antares’ photosphere (its visible surface) would lie between
the orbits of Mars and Jupiter.