The Visible Universe

FAS Astronomers Blog, Volume 31, Number 2.

This is part 1 of a 5-part series on the Universe (The Visible Universe, The Dark Universe, The Microscopic Universe, The Expanding Universe, and The Multiverse).

When we look up at the night sky, we can see clouds. Yes, for most of us interested in astronomy, this is our typical view. So, let’s start again.

When we look up at the night sky on a clear night, we can see stars filling the sky. We also can see several planets, and over time, they slowly move across the sky against the background of stars. If we look hard enough, we also see deep sky objects such as star clusters, nebulae, and galaxies.

The Sun

As we know, there is a star at the center of our Solar System called the Sun. The Sun is technically a yellow dwarf star with a stellar classification of G2V. The Sun is also a middle-aged star. It is around 4 ½ billion years old and we expect it to last another 5 billion years or more.

The Sun is huge. It contains over 99% of all the mass in the Solar System. What that means is over a million Earths could fit inside with room left over. It is also just over 100 times wider than the Earth and 10 times wider than Jupiter.

The Sun has multiple layers ranging from the core through two intermediate layers (the radiative and convection zones) to the photosphere, which is its surface. It also has an “atmosphere” consisting of the chromosphere and corona. The temperature at the core is 27 million degrees (F). At its surface, the temperature drops to around 10,000 degrees (F). Cooler regions called sunspots will appear on the Sun’s surface from time to time. Although it takes eight minutes for light from the Sun to reach us on the Earth, it takes over 100,000 years for light to work its way from the core to the surface of the Sun.

The Sun’s surface exhibits weather, but not in the way we think about weather here on the Earth. There is a steady flow of radiation and charged particles (solar wind) emanating from the Sun. The Sun can also generate huge disturbances, which we see as solar prominences (clouds of hot plasma), solar flares (streams of electromagnetic radiation), and coronal mass ejections (streams of charged particles).

Again, the Sun is a star. It looks big compared to the other stars we see because it is relatively close. The Sun is around 93 million miles from the Earth. The next closest star, Proxima Centauri, is 4.24 light years away – that is around 24 ½ trillion miles. A light year (5.8 trillion miles) is the distance light can travel in a year.

To learn more, see an earlier article about the Sun.

Stars

Stars are big balls of mostly hydrogen gas (or more correctly plasma). They are dense enough and the temperature is high enough so that nuclear fusion reactions take place in their core. Stars are also massive with a huge gravitational force trying collapse it inward. There is a delicate balance between the two. The outward force of the fusion reactions just balances the inward pull of gravity. This equilibrium is maintained for millions, billions, or trillions of years as a star burns.

Stars come in all sizes and many colors from small and cool red dwarfs all the way up to large and hot blue giants. Their spectral class is defined by the Morgan-Keenan System (remember Oh Be A Fine Guy/Girl Kiss Me). They are also classified using something called the Hertzsprung-Russell diagram. Ordinary stars like the Sun are found on the main sequence. Their exact position depends on their size, color, and temperature. Older stars eventually move off the main sequence and become red giants, red and blue supergiants, or white dwarfs.

The Sun is not the largest, nor is it the smallest star. There are many stars (particularly red dwarfs) that are smaller. Some are close to the same size (e.g., Sirius). Several of the stars we can see at night are larger – some much larger (e.g., Rigel and Betelgeuse). The last time I checked, the largest star that we know of is UY Scuti at around 1,700 times the width of the Sun. This is larger than the previous record holder, VY Canis Majoris.

Stars are not all the same age. Younger stars can be found in open clusters such as the Pleiades. Older stars are found in globular clusters such as M13 in the Hercules constellation. Large blue stars burn their hydrogen fuel quickly and might last only a few million years. A star like the Sun will last around ten or so billion years. Small red dwarfs will last for trillions of years.

There are many (many) stars out there. We can probably see over a thousand stars from a dark location. The Milky Way galaxy might have some 200 billion stars. If we assume that an average galaxy has 200 billion stars and there are 200 billion galaxies in the visible universe, then the visible universe has around (200 billion) x (200 billion) stars = 4 x 10²² = 40,000,000,000,000,000,000,000 stars. Ready to count them? 1 … 2 … 3 … It might take a while.

Planets and Exoplanets

Officially, our Solar System has eight planets, five dwarf planets, and over 200 moons. Four of the planets (Mercury, Venus, Earth, and Mars) are rocky terrestrial planets. Four (Jupiter, Saturn, Uranus, and Neptune) are huge gas and ice giants. Two moons (Ganymede and Titan) are larger than the planet Mercury. Then there is Pluto along with four other dwarf planets (Ceres, Haumea, Makemake and Eris). I’m not going to get into a discussion about Pluto here. You can learn much more about Pluto in three previous articles: Discovering Pluto, Pluto, the Kuiper Belt, and the Outer Solar System, and Reclassifying Pluto.

Over the last few decades, astronomers have discovered extra-solar planets (exoplanets) orbiting other stars. The official count is over 5,000 exoplanets. However, the thinking is that most stars have planets, so there are probably trillions and trillions of them out there.

Exoplanets come in different sizes like the planets in our solar system (Mercury-sized, Earth-sized, Neptune-sized, and Jupiter-sized). The exception are the “super-Earths” and “mini-Neptunes”. These planets are larger than the Earth, but smaller than Neptune and, for some reason, we don’t find any like these in our solar system.

Astronomers are searching the sky for more exoplanets. The goal is to find an Earth sized planet in what is called the habitable zone (aka the Goldilocks zone) where it is not too hot and not too cold, but just right for a planet like the Earth. Who knows, “Earth 2.0” might be out there somewhere.

Galaxies

Stars are found in galaxies and our star, the Sun, is located in the Milky Way galaxy.

When we look up from a dark location, we can see a band of light stretching across the sky. That is the Milky Way. We’re actually looking at it edge on from the inside. So, we really can’t directly see what it looks like. But we do know that our Milky Way is a barred spiral galaxy some 100,000 light years across. The Milky Way has a bar at its center and two main spiral arms (Scutum-Centaurus and Perseus) coming off the bar. The Sun is around ½ of the way out from the galactic center in the Orion spur near the minor Sagittarius arm of the Milky Way.

The Milky Way is found in the local group of galaxies along with around 50 or more other members. Our galaxy is one of the two large spiral galaxies in the local group. The other is Andromeda, which is moving toward us and will collide with the Milky Way to create the Milkomeda galaxy in another 4 ½ billion years. It will then take up to 3 billion years for them to merge completely. Remember to duck.

It is not completely clear which came first, galaxies or stars. In any event it is gravity that binds stars together in galaxies. When we look at the cosmic microwave background (radiation left over from the early universe), we see that the universe is not completely smooth. There are patches that are hotter and patches that are colder. Astronomers believe that the hotter patches eventually formed galaxies and galactic clusters. The cooler patches became voids.

Not all galaxies look the same, and they tend to come in three primary shapes (Elliptical, Spiral, or Irregular). The Milky Way and Andromeda are both examples of spiral galaxies. Once they merge, Milkomeda will be an elliptical galaxy. The two satellite galaxies of the Milky Way, the Large and Small Magellanic Clouds, are both irregular galaxies.

Most, if not all galaxies, have supermassive black holes at their center. This includes the Milky Way with Sagittarius A*. The first ever image of our black hole was taken just a few months ago.

When early galaxies were forming, these black holes began pulling in huge amounts of material as they grew larger and larger. The galactic material swirled around the black holes as an accretion disk, but it was so much material that they couldn’t absorb it all at once. The friction from the material falling into the black hole heated up the accretion disk so much that it began to emit high energetic electromagnetic radiation. We see this radiation in the distant part of the universe as extremely bright quasars, and they are evidence of early galaxies being formed.