Mars vs. Antares

Mars will pass 4 degrees north of the bright star Antares the evening of Oct. 19, giving backyard observers a fun show.

Mars and Antares are both red in appearance and look impressive when viewed side-by-side, as this month's positioning will allow.

Mars appears red because of the high iron oxide (rust) content in its soil. Antares looks red because of its surface temperature.

Five degrees is about the width of three fingers held against the sky at arm's length, so Mars and Antares will be closer than this to one another. Look in the southwestern sky near the horizon just after sunset. Bonus: a beautiful crescent moon will appear just to the right of the red planet and star pair.

Antares' name is derived from Greek words meaning "rival of Ares" (Ares is the Greek equivalent to the Roman god Mars) and refers to the objects' similar colors.

Looking at them through binoc ulars allows observers to compare their reddish hues. This month, the comparison is easier because both objects are equally bright.

Antares is a red supergiant in the constellation Scorpio and is often referred to as "the heart of the scorpion." Red supergiants are the largest stars in the universe. Betelgeuse, the orange-red star at Orion's left shoulder, is the same type of star as Antares.

Just like red giants, red supergiants form when a star has used up the hydrogen fuel in its core, and expands out during its helium-burning phase. Unlike red giants, red supergiants have the amount of mass required to continue fusing elements in their cores, all the way up to iron.

Antares is a slow variable star with an average magnitude of 1.09. Its apparent magnitude slowly varies from 0.88 to 1.16.

It is Scorpio's alpha (brightest) star and the 16th brightest star in the night sky overall. Antares is 550 light years from Earth and has a radius of approximately 883 times that of the sun. If Antares were placed in the center of our solar system, its outer surface would lie between the orbits of Mars and Jupiter.

Along with Aldebaran, Spica and Regulus, it is one of the four brightest stars near the ecliptic, which is the path on which the planets travel across the sky - kind of like an equator for the dome of space.

Antares has a secondary companion star, Antares B, that changed from an angular separation of 3.3 arcseconds in 1854 to 2.86 arcseconds in 1990. Arcseconds are an astronomical measurement equaling a tiny fraction of a degree (of angle, not temperature).

The companion star is hard to make out in small telescopes because it is drowned out by Antares' glare, but it can sometimes be seen in 6-inch scopes or larger. The companion looks green, which is probably a contrast effect.

However, Antares B can be observed even with a small telescope for a few seconds during lunar occultations, while Antares itself is hidden by the moon. This is actually how it was discovered, on April 13, 1819.

The orbital period of the two stars around one another is estimated at 878 years.


Oct. 20, 21 - Orionids meteor shower. The Orionids is an average shower producing about 20 meteors per hour at their peak. The first quarter moon will set by midnight. Best views will be to the east after midnight from a dark location far from city lights.

Oct. 29 - Full moon

Oct. 31 - At 2:38 a.m., the Jovian moon Ganymede's shadow touches Jupiter's limb. For the next two hours, the small black dot will traverse Jupiter's south polar regions. Ganymede itself will be visible east of Jupiter, shining against the blackness of space.

Jupiter's innermost major moon, Io, will emerge from behind the planet's eastern limb at 3:07 a.m.


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