Pleiades (M45)

Pleiades (M45)

The Pleiades (M45) as imaged with the Takahashi E-180 Astrograph from Bifrost Observatory. For complete details about this image, see Pleiades (M45). Photo copyright 2012 by Fred Espenak.

The Pleiades, M45 or Seven Sisters is one of the best known star clusters in the entire sky. It has been known since antiquity to cultures worldwide and is easily visible to the naked eye on Winter evenings from the Northern Hemisphere. There are references to the Pleiades in the Odyssey, the Bible, and the Quran, and it is also revered in Hindu mythology. Located in the constellation of Taurus, the Pleiades forms a tiny dipper-shaped asterism in the shoulder of the Bull.

This open cluster is composed of extremely hot and luminous blue stars concentrated within a sphere of about 15 light years in diameter. Over 1,000 stars have been identified as members of the cluster, of which about 24 are visible to the naked eye. The faint reflection nebula visible around the brightest stars is an unrelated dust cloud that the cluster is currently passing through.

The age of the Pleiades is estimated to be between 75 and 150 million years making it quite young compared to the Sun’s age of 4.5 billion years. It is traveling slowly in the direction of Orion, but gravitational interactions with other stars in the galactic neighborhood will eventually disperse the cluster about 250 million years in the future.

Early measurements of the cluster’s distance placed it about 135 parsecs (440 light years) from Earth. However, more precise measurements from the
Hipparcos satellite yield a distance of 118 parsecs (385 light years) by using the parallax of stars in the cluster. Although this 14% decrease in distance may not seem significant, it is important because the distances of nearby clusters like the Pleiades are a first step in calibrating the cosmic distance scale. More recent observations from Hubble support the original larger distance so the subject remains unsettled.

Pleiades (M45)

This two-frame mosaic of the Pleiades (M45) was taken the ASA N12 Astrograph from Bifrost Observatory. For complete details about this image, see Pleiades (M45). Photo copyright 2012 by Fred Espenak.

The 18th-century French comet hunter Charles Messier added the Pleiades to his famous catalog as object M45. The original purpose of the catalog was to identify “fuzzy” comet-like objects (star clusters, nebulae and galaxies) to exclude from his true quest for new comets. Today, the Messier Catalog is recognized as the finest collection of bright deep sky objects by amateur astronomers everywhere.

Now it’s time to go outside on some clear, crisp January evening and view the Pleiades with the naked eye, binoculars or even a small telescope.

Fred Espenak

Andromeda Galaxy

Andromeda Galaxy - M31

The Andromeda Galaxy or M31 is the nearest spiral galaxy to our own Milky Way Galaxy. For complete details about this image, see Andromeda Galaxy. Photo copyright 2012 by Fred Espenak

One of the greatest debates of American astronomy occurred in 1920. It centered on the true nature of the spiral nebulae – pinwheel shaped hazy patches of light visible in the night sky. Harlow Shapley believed they were located relatively nearby inside our own Milky Way. In contrast, Heber Curtis insisted the spiral nebulae are actually ‘island universes’ far outside the Milky Way and are comparable in size and nature to our own galaxy.

Edwin Hubble (namesake of the Hubble Space Telescope) settled the Great Debate by using Cepheid variable stars to measure the distance to the Great Andromeda Nebula – the biggest and brightest the spiral nebulae. Using the 100″ Hooker Telescope, Hubble showed that Andromeda was actually a galaxy like the Milky Way and was located at a distance of about 1.5 million light-years.

Modern measurements of the Andromeda Galaxy place it at a distance of approximately 2.5 million light-years from Earth. Located in the constellation Andromeda, the Andromeda Galaxy can be seen with the naked eye from a dark moonless sky as a hazy oval patch about twice the diameter of the Full Moon. During December evenings, Andromeda passes overhead around 8 pm from mid-northern latitudes. A pair of 7×50 binoculars provides a good view of the galaxy.

Constellation of Andromeda

The constellation Andromeda was known to the ancients as the Chained Maiden. In this wide angle view of the constellation, the Andromeda Galaxy is visible as a hazy oval. For complete details about this image, see Andromeda. Photo copyright 2012 by Fred Espenak

The 18th century French comet hunter Charles Messier included the Andromeda Galaxy as entry M31 in his famous catalog of deep sky objects. In August of 1764, Messier gave the following description:

“The beautiful nebula of the belt of Andromeda, shaped like a spindle; M. Messier has investigated it with different instruments, and he didn’t recognise a star: it resembles two cones or pyramides of light, opposed at their bases, the axes of which are in direction NW-SE; the two points of light or the apices are about 40 arc-minutes apart; the common base of the pyramids is about 15 (arc-minutes).”

The Andromeda Galaxy is the closet spiral galaxy to the Milky Way and it resembles how our own galaxy would appear from a great distance. Recent research indicates that M31 was formed from the collision and merger of two smaller galaxies about 10 billion years ago. Some 2 to 4 billion years ago, M31 experienced a close encounter with the Triangulum Galaxy (M33). The encounter triggered high levels of star formation in M31, and disturbed M33’s outer disk.

M31 is accompanied by two satellite elliptical galaxies known as as M32 (NGC 221) and M110 (NGC 205). Both are easily seen and photographed with amateur telescopes. Research with larger telescopes has identified a total of 14 dwarf satellite galaxies gravitationally associated with M31.

Andromeda Galaxy - M31

This close-up of the Andromeda Galaxy clearly shows its two satellite elliptical galaxies known as as M32 (NGC 221) and M110 (NGC 205). The image was shot with a 12″ ASA telescope from Bifrost Observatory in Portal, Arizona. For complete details, see Andromeda Galaxy. Photo copyright 2012 by Fred Espenak

The Andromeda Galaxy and its satellites, along with the Milky Way Galaxy and the Triangulum Galaxy (M33), are all part of the Local Group of about 54 nearby galaxies. The gravitational center of the group is located somewhere between the Milky Way and M31.

The Andromeda Galaxy is approaching the Milky Way at about 2 billion miles per year and is expected to collide with us in about 4.5 billion years. Although the outcome is uncertain, it is likely that the galaxies will merge to form a giant elliptical galaxy. This is a common event among interacting galaxies. In any case, the collision will take place near the end of the Sun’s expected lifetime.

What an amazing view it will be when the Andromeda Galaxy dominates the night sky a few hundred million years before the collision! (see Andromeda Collision for a preview)

Fred Espenak