From Hubble: a grand new view of M9

Photo: M9 globular star cluster. Hubble image by NASA & ESA.
The stars of the M9 globular cluster. Photo credit: NASA & ESA.

The NASA/ESA Hubble Space Telescope has produced the most detailed image so far of Messier 9 (M9), a globular star cluster located close to the center of the galaxy. This ball of stars is too faint to see with the naked eye, yet Hubble can see over 250,000 individual stars shining in it.

M9, pictured here is a roughly spherical swarm of stars that lies around 25,000 light-years from Earth, near the center of the Milky Way, so close that the gravitational forces from the galactic center pull it slightly out of shape. Globular clusters are thought to harbor some of the oldest stars in our galaxy, born when the Universe was just a small fraction of its current age. As well as being far older than the Sun —around twice its age— the stars of M9 also have a markedly different composition, and are enriched with far fewer heavier elements than the Sun.

In particular, the elements crucial to life on Earth, like oxygen and carbon, and the iron that makes up our planet’s core, are very scarce in M9 and clusters like it. This is because the Universe’s heavier elements were gradually formed in the cores of stars, and in supernova explosions. When the stars of M9 formed, there were far smaller quantities of these elements in existence.

M9, as its name suggests, was discovered by the great French comet hunter Charles Messier in 1764. Even through the most advanced telescopes of the day, none of the stars in the cluster could be seen individually. Messier, seeing only a faint smudge, therefore classified the object as a nebula –or “cloud” in Latin– and put it on his list of objects that looked like but were not comets. It was only later in the 18th century that astronomers, most notably William Herschel, began to spot stars within the cluster.

The contrast between Messier’s equipment and the tools at the disposal of today’s astronomers is stark. Hubble’s image, the highest resolution image yet made of M9, is able to resolve individual stars, right into the crowded center of the cluster. Over 250,000 of them are neatly focused on the detector of Hubble’s Advanced Camera for Surveys, in an image which covers an area of sky no bigger than the size of the head of a pin held at arm’s length.

As well as showing the individual stars, Hubble’s image clearly shows the different colors of the stars. A star’s color is directly related to its temperature — counter-intuitively, perhaps, the redder it is, the cooler it is; and the bluer it is, the hotter. The wide range of stellar temperatures here is clearly displayed by the broad palette of colors visible in Hubble’s image of M9.

A summery triangle

Image: Map depicting summer's triangle of stars.
The Summer Triangle - by James Guilford using Stellarium and Adobe Photoshop

by William Murmann

During the summer months, stargazers can see the famous Summer Triangle almost directly overhead. The triangle is a giant asterism created by drawing imaginary lines between three bright stars — Deneb in the constellation Cygnus, Vega in the constellation Lyra, and Altair in the constellation Aquila.

Deneb is a blue-white super-giant that is almost 200 times larger than the Sun and 60,000 times brighter. At 1,500 light years distant, it is one of the most luminous stars known and is the farthest first-magnitude star from Earth. It has a solar wind that is 100,000 times faster than the solar wind from the Sun.

Vega is a blue-tinged white star that is about 25 light years away. It is twice the mass of the Sun and about 37 times brighter. At 16,000-degrees F, its surface temperature is almost twice as hot as the surface of the Sun. With the exception of the Sun, Vega is the first star to have been photographed. In 14,000 years, it will replace Polaris as our north star.

Altair, at a distance of 16.9 light years is about 1.5 times larger than the Sun, and is one of the closest stars visible to the naked eye. Altair spins on its axis at about 640,000 mph and completes a full revolution every 6.5 hours. The Sun in comparison takes about 25 days to complete one revolution, as measured at its equator. Altair is spinning so fast that its north and south poles are pushed in, giving the star an oblate appearance.

The Summer Triangle was first described as a triangle by Austrian astronomer J.J. Littrow in his atlas in 1866. German astronomer Johann Bode connected the stars in a map in 1816, but did not label the asterism.