September Guest Post: Stars in Our Skies

Authors: Althea Crabtree and Madison Varner (Sisters High School)

M30 is a dense globular star cluster located 30,000 light-years away in the constellation of Capricornus. Image Credit: NASA/ESA

This September we are featuring a collection of stars from the zodiac known as Capricornus. When translated through Latin, the constellation represents a sea goat. This mythical creature was associated with the god Enki in Babylonian mythology, and later with the Greek deity Pan. In Greek mythology, the constellation is sometimes seen as Amalthea, a goat with a broken horn, which was later transformed into the “horn of plenty,” cornucopia.

To find Capricornus in the sky requires exceptionally clear, dark skies because its constituent stars are faint. When conditions are in your favor, look towards the fourth quadrant of the southern sky. To some observers, the constellation resembles a primitive boat. Bordering constellations include Aquarius, Aquila, Microscopium, Piscis Austrinus, and Sagittarius.

The constellation's brightest star is known as Delta Capricorni, also called Deneb Algedi. It is a multiple star system with an apparent magnitude of 2.81, located at a distance of 39 light-years from Earth. The primary member of the group is a blue-white subgiant star that has about twice the sun’s mass and radius and is 8.5 times more luminous.

While there are several deep sky objects in Capricornus, all but one of them are dim galaxies, mostly beyond the range of most backyard telescopes. The lone exception is M30 (NGC 7099), a very fine globular cluster. It is a particularly dense collection of stars, perhaps caused by a core collapse, a common end-stage for globular clusters that dramatically increases the central star density. Positioned approximately 30,000 light-years away, this object is about 90 light-years across in size and believed to be 13 billion years old.

In addition to Capricornus, September also marks the beginning of "Milky Way Season", the period of the year most ideal for seeing the Milky Way in the Northern Hemisphere. The galatic core, located in the constellation of Sagittarius, is the part that is most recognizable and bright due to the high concentration of stars near the center of our galaxy. To fully experience the grandeur of the Milky Way that arches overhead, observe from a dark location away from lights, whether they be artificial or otherwise. 

An important event not to be overlooked this month is the arrival of autumn in the Northern Hemisphere. This year it will occur on Monday September 22 at 11:19 a.m. PDT when the southward-trending sun crosses the celestial equator, which is just the astronomical extension of Earth’s equator.

After reaching opposition on September 21, Saturn will officially become an evening object and be visible all night. The morning sky will be lit up by the brighter planets Jupiter, rising about 1 a.m. local time, and Venus making its appearance around 4:30 a.m. Early in the month it may be possible to glimpse Mercury low on the eastern horizon before glare from the sun becomes too great. Likewise, Mars is hidden from view by the encroaching sun.

If you have been watching the beautiful moon orbit us throughout this summer, and wish to continue this in September, be sure to admire the Full Corn Moon on September 7, last quarter on the 14th, new moon on the 21st, and first quarter on the 29th. The full harvest moon will occur in October this year. 

To ensure that faint constellations, such as Capricornus, continue to be visible, we need to minimize light at night. Sisters is now an international dark sky community, which means we have committed to do all we can to limit light pollution. You can help achieve this goal by turning off lights when you are not using them. This is especially important for outdoor lights, but indoor lights can cause light pollution too.

Remember to get out there and enjoy our dark skies!