Saturn, the Andromeda Galaxy, and the Tarantula Nebula

Observatory / Course Archives / ASTR 110 Fall 2019 / Aupperlee

Saturn

Saturn is the sixth of eight planets orbiting around the Sun. In order for an object to be considered a planet it must:

1. Orbit only the Sun
2. Be large enough that its gravity forces it into a spherical shape
3. It must have cleared its orbit of objects of similar size

Saturn is the second largest planet in the solar system in terms of size and mass. Its diameter is 9 times that of Earth's. Though it has almost 60% of Jupiter's volume, it is only one-third of its mass. It has the lowest density of any known object in the solar system. Saturn is 9.5 times as far away from the Sun as Earth is, and its orbit is approximately 29.5 years. Saturn is nearly a billion miles away from the Sun (933,248,213 mi to be exact). It only takes 10.7 hours for it to rotate on its axis.

Saturn is a gas giant with an iron, nickel, and iron core, followed by metallic hydrogen layer, then a layer of liquid hydrogen and helium, and finally a layer of gaseous hydrogen and helium. Saturn's pale-yellow hue is because of ammonia crystals in its outer atmosphere. To the naked eye, it appears a bright yellowish point. Saturn's rings, the most visible and prominent of all the planets, are composed almost entirely of rock and ice. Their origin is still unknown, though the most popular hypothesis is that they are made from broken up moons, asteroids, and comets. Though the rings extend out from the planet at distances as great as 175,000 miles, the rings are often only about 30 feet thick. Saturn's moon (Saturn as a total of 82 confirmed moons) Titan is unique among moons as it has a stable atmosphere, one that is denser than all of the terrestrial planets excepting Venus. Saturn was first viewed through a telescope in 1610 by Galileo.

Andromeda Galaxy

The image above is of the Andromeda Galaxy. A galaxy is a collection of stars, a lot of dust and gas if it is a spiral galaxy, and a ton of dark matter that is all held together in orbit by gravity. Like two-thirds of all galaxies, the Andromeda Galaxy is a spiral galaxy. This means that it has a central bulge of older stars (and likely, a supermassive black hole at its center), spiral arms of stars (young and old), and a halo of lone older stars, large amounts of hot gas, and a massive amount of dark matter. The Andromeda Galaxy (also known as M31) is a spiral galaxy and the closest normal-galaxy to the Milky Way (technically, the Magellanic clouds are closer, but they are identified as irregular galaxies). It is 200,000 lightyears across and for a long time, scientists believed that it was a part of the Milky Way, thus naming it the Andromeda Nebula. It was not until the 1920’s that it was concluded to be a separate galaxy by Edwin Hubble. Even though it is 2.48 million lightyears away, the Andromeda Galaxy appears very bright in the night sky and is even visible to the naked eye, even in areas with moderate light pollution. The earliest mention of the galaxy was made by Persian astronomer Al-Sufi in the year 964 AD.

The Andromeda Galaxy has approximately 1 trillion stars. It is estimated to have formed 10 billion years ago, likely the product of several protogalaxies colliding. The yellowish light coming from the center (bulge) of the galaxy is mostly older, redder stars (Type II). The arms of Andromeda have bluer light, indicating younger stars (Type I), but it is believed that star formation in the arms has almost completely ceased. Andromeda’s huge halo contains an impressive amount of hot gas, nearly half the mass of the stars in the galaxy. The halo also contains immense amounts of dark matter and reaches as far as 1 million lightyears out from the central body of the galaxy. It is predicted that the Andromeda Galaxy and the Milky Way will collide in 4.5 billion years, likely forming a large elliptical galaxy. Andromeda has “devoured” other galaxies in the past. Its large and complex halo indicates that Andromeda has taken on many other galaxies and perhaps even larger ones.

Tarantula Nebula

The above image is of the Tarantula Nebula. A nebula, a huge cloud of interstellar gas and dust, mostly hydrogen and helium, often comes from supernovae (the explosion of a dying star). Nebulae, though they often mark the death of a star, are also the birthplace of many stars. Over time, the gas and dust of a nebula can condense to form new stars and because of this, nebulae are often called “star nurseries.” The Tarantula Nebula, also known as 30 Doradus (because it is a part of the Doradus constellation), is 160,000 lightyears away and part of the Large Megellanic Cloud, a satellite galaxy to the Milky Way. It was discovered in 1751 by Nicolas-Louis de Lacaille. It is an impressive 1000 lightyears across, the brightest star forming area in the Local Group. A collection of young, hot stars within the Tarantula Nebula ionize its gas, causing the gas to illuminate. Because there is little cosmic dust between Earth and the Tarantula Nebula, it is very easily observed. It gets its name from several of its bright patches that slightly resemble the legs of a tarantula.

The nebula’s reddish tint comes from the glow of ionized hydrogen gas. The blue light of the Tarantula comes mostly from a star cluster called NGC 2070. This cluster is quite young and contains some of the most luminous and massive stars ever discovered. Some of the gas visible in the Tarantula Nebula comes from the star cluster Hodge 301, in which approximately 40 stars exploded as supernovae, spreading gas out into the nebula.