The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived radioactive elements such as thorium-232 and uranium-238, then comparing the results to estimates of their original abundance, a technique called nucleocosmochronology.Continue reading “nucleocosmochronology”
The Andromeda Galaxy , also known as Messier 31, M31, or NGC 224, is a spiral galaxy approximately 780 kiloparsecs (2.5 million light-years) from Earth, and the
nearest major galaxy to the Milky Way. Its name stems from the area of the sky in which it appears, the constellation of Andromeda.
Andromeda ‘Smudge’ – nebulous smear – you can see it as Abd al-Rahman al-Sufi saw it.
Around the year 964, the Persian astronomer Abd al-Rahman al-Sufi described the Andromeda Galaxy, in his Book of Fixed Stars as a “nebulous smear”. Star charts of that period labeled it as the Little Cloud.
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age.
More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist.
Gravitational Attraction
They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters.
Globular Clusters
Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer.
Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.
Finding the stars with the naked eye – astronomy starts by looking up at the sky without any optical prejudice.
Sagitta is a dim but distinctive constellation in the northern sky. Its name is Latin for “arrow”, and it should not be confused with the larger constellation Sagittarius, the archer. Although Sagitta is an ancient constellation, it has no star brighter than 3rd magnitude and has the third-smallest area of all constellations (only Equuleus and Crux are smaller). It was included among the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations defined by the International Astronomical Union. Located to the north of the equator, Sagitta can be seen from every location on Earth except within the Antarctic circle.
Norma is a small constellation in the Southern Celestial Hemisphere between Scorpius and Centaurus, one of twelve drawn up in the 18th century by French astronomer Nicolas Louis de Lacaille and one of several depicting scientific instruments.
Its name is Latin for normal, referring to a right angle, and is variously considered to represent a rule, a carpenter’s square, a set square or a level. It remains one of the 88 modern constellations.
Constellations are hard to find with only a short time of dark sky. The Milky Way is a great view at this time, if you know where to look. I like to see it where I live because the view is usually quite bad with light pollution.
Before Galileo, it was thought that all bright objects in the sky were either the planets in the Solar System, moons, comets, or stars. Until the beginning of the twentieth century, astronomers did not know the size of the Universe, but speculated it to be about as big as the Milky Way.
In 1920, at the National Academy of Science, there was a big debate between Harlow Shapley and Heber D. Curtis on whether nebulae are small globular clusters surrounding the Milky Way, or separate galaxies located farther away. Nothing was resolved at the debate; neither side was able to provide conclusive evidence to prove their side correct over their opponent.
In 1923, Edwin Hubble resolved the matter with a photograph that he took of the Andromeda Galaxy. What he found in his photograph was a very bright light source pulsing at a certain rate, a Cepheid variable, located outside the Milky Way. This can be used to determine the distance to it.
Hubble proved that the Universe was full of galaxies, and disproved that the Milky Way was the extent of the Universe. There are many types of galaxies in the Universe, elliptical, barred spiral galaxies; they vary in shape and size, but on average spiral galaxies are the most abundant.
The majority of mass in galaxies is made up of dark matter, a substance which is not directly observable, and might not interact through any means except gravity.
Halo stars are typically much older and have much lower metallicities (that is to say, they are almost exclusively composed of hydrogen and helium) than disk stars.
Many disk galaxies have a puffed up outer disk (often called the “thick disk”) that is composed of old stars.
Globular clusters are typically old and metal-poor as well, but there are a few that are not nearly as metal-poor as most, or have some younger stars.
High-velocity clouds, clouds of neutralhydrogen are “raining” down on the galaxy, and presumably have been from the beginning (this would be the necessary source of a gas disk from which the disk stars formed).
Galaxies come in a great variety of shapes and sizes (see the Hubble sequence), from giant, featureless blobs of old stars (called elliptical galaxies) to thin disks with gas and stars arranged in highly ordered spirals.
The majority of giant galaxies contain a supermassive black hole in their centers, ranging in mass from millions to billions of times the mass of our Sun. The black hole mass is tied to properties of its host galaxy.
Many of the properties of galaxies (including the galaxy color–magnitude diagram) indicate that there are fundamentally two types of galaxies. These groups divide into blue star-forming galaxies that are more like spiral types, and red non-star forming galaxies that are more like elliptical galaxies.
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