June 2009 Sky from the Keeble Observatory
Last month we wrote about the “dipole anisotropy” in the cosmic microwave background
radiation field. This results from our motion through the universe, making half
the sky a fraction of a degree cooler and the other half a fraction warmer than
the 2.725 K temperature of the sky. If your eyes were sensitive to microwaves, it
would look something like this:
(image courtesy of NASA/COBE Science Team), which represents the
entire sky projected in “galactic coordinates” – the plane of the Milky Way bisects
the long axis of the image. This represents about four years’ data collection by
a satellite called the Cosmic Background Explorer, aka COBE, which was launched
The red regions are cooler, the blue are hotter (red shift and blue shift due to
the Doppler effect) by about .003 Kelvin compared to the overall average temperature.
A more recent experiment, the Wilkinson Microwave Anisotropy Probe (WMAP) accumulated
five years of data at a much finer angular resolution. After subtracting the dipole
anisotropy and the microwave emission from our own Galaxy, their results looked
like this (image courtesy NASA/WMAP Science Team):
The “wrinkles” in this image represent temperature difference on the order of .0002
K, or fluctuations of roughly one part in 10,000 about the average. What do these
First, they tell us that there was already structure in the universe at the time
of decoupling of matter from radiation, i.e. when the universe became transparent.
It is this structure which led ultimately to the formation of galaxies and stars.
Analysis of the angular scale of these fluctuations constrains our models of the
early universe. They tell us that the ordinary matter which surrounds us and of
which we are made represent only about 4% of everything that’s actually there. The
rest consists of still mysterious stuff called “dark matter” and “dark energy.”
Calling them dark means that they don’t interact with light, but we can still tell
of their presence by their gravitational interactions. We need dark matter to hold
the Galaxy together, for example, since the mass of all the stars and gas and dust
is insufficient. Dark energy is what drives the accelerating expansion of the whole
universe. Their exact nature remains the subject of ongoing research, both theoretical
Lunar phases for June: Full Moon on the 7th, at 2:12; Last Quarter
on the 15th, at 6:15 6m; New Moon on the 22nd, at 3:35 pm;
First Quarter on the 29th, at 7:28 am.
Early risers will have an array of planets to watch – Mercury, Mars, Venus, and
Jupiter are all up in the predawn sky. To the east, the brightest object before
sunrise will be Venus, only a few degrees above and to the right of red Mars. Venus
will be at its greatest apparent separation (known as maximum western elongation)
from the Sun on the 5th, and then begin slowly drifting back closer.
They’ll get closer as the month progresses. Below and to the right of this bright
pair, you may see Mercury low on the horizon. It will climb higher as July approaches.
Jupiter is already high to the south at sunrise. With a modest telescope you may
see Neptune close by. Both will drift toward the southwest and lower by month’s
end. Evening planet watchers will have magnificent Saturn for the whole month. Look
high to the south as evening twilight shifts to night. Early in June you’ll be able
to watch Saturn for about six hours until it sets; by the end of the month it emerges
from twilight more to the southwest, and sets about 4 hours later.
An overhead view at mid-month, about two hours after sunset, finds the constellation
Bootes (the Herdsman) at zenith. The brightest star in this large constellation,
Arcturus (or alpha Bootes) lies nearly 20 degrees to the south-southwest. Further
to the west we find Saturn, nearly thirty degrees above and to the left of Regulus
(alpha Leo). Turning toward the northwest, you will see the familiar “Big Dipper”
of Ursa Major, with its handle (or tail) almost vertical. Ursa Minor, the “Little
Dipper” lies due north, appearing to stand on its handle above its brightest star
Polaris. To the east-northeast, the bright star you will notice about 45 degrees
above the horizon is Vega (alpha Lyrae). It lies above the constellation Cygnus,
here with its long axis parallel to the horizon, with the head of the Swan (Albireo)
at the southern end and Deneb (alpha Cygni) toward the north.