PISGAH
ASTRONOMICAL
RESEARCH
INSTITUTE
Dr. Bob Hayward
Astronomer/Educator
Graphics from TheSky
Software Bisque
Mountain Skies
February 8, 2016
ORION IS OUR GUIDE TO FINDING THE DOG STAR
The stars: It’s hard to talk about the winter stars without talking about Orion the hunter. He is
the most spectacular constellation in the entire sky and, as we’ve seen in past columns, can be used to
find other objects and patterns in the sky. Orion can be found high in the east as the sky darkens. The
three stars in a row marking his belt are
fairly obvious. To the north of these are
two more that mark his shoulders and two
below the belt in his legs. Note the colors
of the stars in Orion. A reddish colored
star named Betelgeuse (sometimes called
Beetlejuice) marks his right shoulder.
Rigel, on the opposite side of the
constellation, is quite blue in color. Stars
have different colors, not so much due to
their compositions, but more because of
their temperatures.
Hot stars like Rigel
might be 20,000 degrees Fahrenheit or
more
while
“cool”
supergiants
like
Betelgeuse might be only 5 or 6,000
degrees.
Extend the belt of Orion down to the southeast…to the Dog Star, Sirius. Note its white color; its
temperature is about 10,000 degrees Fahrenheit, intermediate between Rigel and Betelgeuse. Sirius is
actually a double star although it takes a telescope of moderate size to find the companion. The
companion star, known as “Sirius B,” is a faint white dwarf star, one of a class of stars that typically
have the mass of a star similar to the sun but are only the size of a planet like the Earth. Thus, they are
very dense with surface gravities perhaps 30,000 times that of the Earth. Astronomers understand they
are old stars that are at the ends of their energy producing lifetimes.
Sirius (or “Sirius A”) and Sirius B orbit each other around their common center of gravity once
each fifty years. Because Sirius A is so much brighter, it is hard to spot Sirius B when they are close
together. However, their mutual orbits are very elliptical and they were at their closest point in 1994.
Thus, we have entered a period of time when they are appearing progressively farther apart; for the
next 25 years or so it will be much easier to spot the companion with a moderate size telescope.
The planets: Where have all the bright planets gone? We don’t see any of them in the evening
skies nowadays. Due to their motions around the sun coupled with the fact that we are observing from
a moving platform ourselves, all of the planets are now forming a line in the predawn skies. The
brightest of these is, of course, the brilliant Venus. Venus has been our “Morning Star” since last
summer but it is now standing closer to the sun each morning as it swings around behind our star on
June 6. But now, it still is a guiding beacon to guide us to the other classical planets.
Mercury is now below Venus and to its left in the morning twilight. If you don’t see it at first, wait
a few minutes; Mercury doesn’t rise tomorrow morning until around 6 a.m. But, that still almost 90
minutes before the sun so we can spot it in the morning southeastern twilight well before the sky is too
bright. It is, however, much fainter than Venus. Above and to the right of Venus you will spot the
ringed planet Saturn although, of course, the rings are not apparent without the aid of a telescope. Be
careful. To the right and a bit below Saturn is the bright red star Antares, the heart of the scorpion.
This is NOT Mars. In fact, its very name translates into “Rival of Mars” due to its similarity in brightness
and color to the red planet. But, Mars is close by. Look even further to the right and up, due south, and
you will find it with its distinctive red coloration.
Farther up this line in the sky we’re following is the bright star Spica in Virgo. This, of course, is
not a planet. But, on beyond Spica, high a little to the west of south is the second brightest planet of
them all, the mighty king planet Jupiter. Since Jupiter is farther to the west than the other planets, we
know it must be rising earlier. Such is the case as tonight Jupiter is rising about 8:30 p.m. Realize that
due to the motion of the earth around the sun, Mars, Saturn and Jupiter all rise earlier each night and it
won’t be long before they will be in our evening skies. Venus and Mercury, however, have such swift
motions of their own that the same is not true of them. Mercury will be in our predawn skies until late
this month. We’ll lose Venus by early May and, even then, it will be very low in the morning sky.
Celestial Calendar:
February 8, 9:39 a.m. EST – New Moon
February 15, 2:46 a.m. EST – First Quarter Moon
February 16 – The Sun moves from Capricorn the sea-goat into Aquarius the water-bearer.
PARI is a public not-for-profit public foundation established in 1998. Located in the Pisgah National
Forest southwest of Asheville, NC, PARI offers educational programs at all levels, from K-12 through postgraduate research. For more information about PARI and its programs, visit www.pari.edu. Follow PARI on
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For further information or questions about this Mountain Skies column, contact Dr. Bob Hayward at
rhayward@pari.edu. Graphics produced with TheSky Astronomical Software, Software Bisque.