The Demon Star Dims, and Morning Mars meets the Moon and Vesta — and its Stellar Sister, Too!
Watch the Demon Star Brighten
Everyone has seen stars twinkle. The flickering is produced by turbulence in Earth’s atmosphere that temporarily deflects the narrow beams of starlight before they reach our eyes. But a large percentage of stars actually vary in the amount of light they emit, resulting in changes in their visual brightness on a timescale of hours, days, or even years. There are different reasons for the variations. One variable star’s changes are particularly easy for everyday folks to see. It’s the “Demon Star”, Algol.
Algol is a medium bright, blue-white star in the northern constellation of Perseus (the Hero). This time of year, Perseus sits near the zenith (i.e., directly overhead) as soon as it’s dark. It is located roughly between the Pleiades cluster in Taurus (the Bull) and the W-shaped constellation of Cassiopeia (the Queen). As the evening rolls on, Perseus descends in the western part of the sky, setting about 4 am local time.
Perseus’ brightest star is Mirfak. It’s hard to miss as it’s brighter than any other nearby star, except yellowish Capella, which is about 1.4 fist diameters above it. Algol is the second brightest star in Perseus. It sits 9° (or a fist’s diameter) to Mirfak’s lower left.
The name Algol comes from the expression “ra’s al-ghul”, which means “the head of the ghoul. And yes, that’s the same Ras Al-ghul used by the character in the DC comics! In the constellation, the star represents the gorgon Medusa’s blood-soaked severed head being carried home by Perseus.
Most of the time, Algol shines with a steady light. (It’s about visual magnitude 2.1) But every 2 days, 20 hours, and 49 minutes, Algol drops in brightness (to visual magnitude 3.4). This occurs because the orbit of a smaller, dimmer companion star around the primary star is oriented edge-on to Earth, and once during every orbit it crosses between us and the star, cutting off some of its light for 10 hours. This set up is called an eclipsing binary system. At its minimum, Algol is about equal in brightness to a fainter star located two finger widths to its lower left, Gorgonea Tertia “third star of the gorgon”.
If you start watching Algol near its minimum brightness, you can see start as equal in brightness to Gorgonea Tertia, and then slowly brighten to its regular intensity over the course of the evening. This week, Algol reaches its minimum brightness at the convenient times of 9 pm EST on Sunday, February 4 and at 5:59 pm EST on Wednesday. This process has been happening, and will continue, “forever”, in human terms. I might highlight more good opportunities to see the change in a future Skylights. Let me know if you see it!
For about half an hour after dusk during the two week period between now and the new moon on February 15, look west-southwest for a broad wedge of faint light rising from the horizon and centered on the ecliptic. This is the zodiacal light — reflected sunlight from interplanetary particles of matter concentrated in the plane of the solar system. Try to observe from a location without light pollution, and don’t confuse the zodiacal light with the brighter Milky Way to the northwest.
The Moon and Planets
This is the week of the lunar month when the moon swings towards the morning sun, leaving the evening skies darker. Tonight (Sunday) the still quite bright waning gibbous moon will rise in late evening near the bright star Spica in Virgo (the Maiden). By Wednesday morning, the moon will have waned to Last Quarter (half illuminated on the left sunward side). On that day, it will rise about 12:45 am local time and appear about a palm’s width above bright Jupiter in Libra (the Scales). As dawn approaches, the sky’s rotation will carry the moon to Jupiter’s upper right, and form a triangle with the nearby prominent star Zubenelgenubi. The trio of objects will reach the southern sky by dawn.
On Thursday morning, the moon will hop lower to sit to Jupiter’s left, roughly between that planet and dimmer reddish Mars. On Friday morning, the waning crescent moon will continue its eastward passage through the pre-dawn planets, passing 4° (about four finger widths) to the upper left of Mars in the sky above the constellation of Scorpius (the Scorpion). Look for the pairing low in the southeast between about 5:30 and 6:30 am local time. Meanwhile, the major asteroid (4) Vesta will be positioned only 1.5° to the upper left of the moon. By the way, the “(4)” in the asteroid’s name refers to the fact that Vesta was the fourth asteroid discovered — way back in 1807.
Completing its tour through the pre-dawn planets, on Friday morning, the old crescent moon will pass only two finger widths above the yellowish planet Saturn, which rises about 5 am local time this week. Look for the pair of objects low in the southeastern sky between 5 and 6:30 am local time.
Mars does something interesting this week. On the mornings surrounding Saturday, February 10, the slow eastward orbital motion of the red planet will carry it across the sky only 5° above its visual twin, the red star Antares. The brightest star in the constellation of Scorpius, Antares’ name means “Rival of Mars”. While almost exactly the same visual color and brightness as Mars, the star is about 553 light-years from Earth — while Mars is only 13 light-minutes from Earth!
Jupiter, Mars, and Saturn will continue to rise earlier, and climb higher, every morning during the next few months, eventually transitioning into evening objects in late March, mid-June, and mid-May respectively.
Astronomy Skylights for this week (from February 4th, 2018) by Chris Vaughan.
Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.