Posted on by Jean Tate

Barred Spiral Galaxy

A barred spiral galaxy, from the Galaxy Zoo 2 tutorial (How to Take Part)

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As its name implies, a barred spiral galaxy is a spiral galaxy with a bar through the center.

Hubble introduced the ‘tuning fork’ scheme for describing the shapes of galaxies (“morphologies” in astronomer-speak) in 1936. In this, the two arms of the fork are barred spirals (from SBa to SBc) and spirals without bars (from Sa to Sc); the S stands for spiral, B for ‘it’s got a bar’, and a/b/c for how tightly wound the spiral arms are. This was later extended to a fourth type, SBm and Sm, for irregular barred spiral galaxies which have no bulge.

In 1959, Gérard de Vaucouleurs extended the scheme to the one perhaps the most commonly used by astronomers today (though there’ve been some mods since). In this scheme spirals without bars are SA, and those which have really weak bars are SAB; barred spirals remain SB. He also added a ‘d’ (SAd, SBd), and a few other things, like rings.

About half of spiral galaxies are barred; examples include M58 (SBc), M61 (SABbc), the Large Magellanic Cloud (LMC, Sm), … and our own Milky Way galaxy!

The bars are mostly stars (usually), unlike spiral arms (which have lots of gas and dust besides stars). The formation and evolution of bars is an active area of research in astronomy today; they seem to form from close encounters of the galaxy kind (galaxy near-collisions), funnel gas into the central bulge (where the super-massive black holes there snack on it), and are sustained by the same density waves which keep the arms alive.

Why not join the Galaxy Zoo project, and have some fun classifying spiral galaxies into whether they have bars or not (and getting to see some amazing sights too)?

Hubble Early Release Observation of Barred Spiral NGC 6217, Two Galaxies Walk Into a Bar…, and The Milky Way Has Only Two Spiral Arms; just some of the Universe Today stories on barred spiral galaxies.

Astronomy Casts featuring barred spiral galaxies include The Story of Galaxy Evolution, and Galaxies.

Posted on by John Carl Villanueva

Structure of the Universe

Galaxy cluster Abell 85, seen by Chandra, left, and a model of the growth of cosmic structure when the Universe was 0.9 billion, 3.2 billion and 13.7 billion years old (now). Credit: Chandra

[/caption]The large-scale structure of the Universe is made up of voids and filaments, that can be broken down into superclusters, clusters, galaxy groups, and subsequently into galaxies. At a relatively smaller scale, we know that galaxies are made up of stars and their constituents, our own Solar System being one of them.

By understanding the hierarchical structure of things, we are able to gain a clearer visualization of the roles each individual component plays and how they fit into the larger picture. For example, if we go down to the world of the very small, we know that molecules can be chopped down into atoms; atoms into protons, electrons, and neutrons; then the protons and neutrons into quarks and so on.

But what about the very large? What is the large-scale structure of the universe? What exactly are superclusters and filaments and voids? Let’s start by looking at galaxy groupings and move on to even larger structures.

Although there are some galaxies that are found to stray away by their lonesome, most of them are actually bundled into groups and clusters. Groups are smaller, usually made up of less than 50 galaxies and can have diameters up to 6 million light-years. In fact, the group in which our Milky Way is a member of is made up of only a little over 40 galaxies.

Generally speaking, clusters are bunches of 50 to 1,000 galaxies that can have diameters of up to 2-10 megaparsecs. One very peculiar property of clusters is that the velocities of their galaxies are supposed to be too high for gravity alone to keep them bunched together … and yet they are.

The idea that dark matter exists starts at this scale of structure. Dark matter is believed to provide the gravitational force that keeps them all bunched up.

A great number of groups, clusters and individual galaxies can come together to form the next larger structure – superclusters. Superclusters are among the largest structures ever to be discovered in the universe.

The largest single structure to be identified is the Sloan Great Wall, a vast sheet of galaxies that span a length of 500 million light-years, a width of 200 million light-years and a thickness of only 15 million light-years.

Due to the limitations of today’s measuring devices, there is a maximum level to which we can zoom out. At that level, we see a universe made up of mainly two components. There are the threadlike structures known as filaments that are made up of isolated galaxies, groups, clusters and superclusters. And then there are vast empty bubbles of empty space called voids.

You can read more about structure of the universe here in Universe Today. Want to read about the cosmic void: could we be in the middle of it? We’ve also written about probing the large scale structure of the universe.

There’s more about it at NASA. Here are a couple of sources there:

Here are two episodes at Astronomy Cast that you might want to check out as well:

Sources: NASA WMAP, NASA: Sheets and Voids