The Milky Way Has Only Two Spiral Arms

by Fraser Cain on June 3, 2008

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If you were stuck inside your house, you’d never know what it looks like from the outside. That’s the situation with the Milky Way. We’re inside it, so we don’t really know what its structure looks like. There are other examples of grand spirals that we can see, but this is like seeing other houses outside your window; you just can’t be sure. Astronomers have developed a detailed map of the Milky Way, and realized that they were giving our home galaxy too many arms; it’s only got 2, and not 4 like astronomers originally thought.

The new revelation was made possible thanks to NASA’s Spitzer Space Telescope, which sees in the infrared spectrum, and can peer though the gas and dust that obscures the plane of the Milky Way.

Previous maps of the Milky Way were first developed in the 1950s, when astronomers used radio telescopes to trace out the spiral arms of our home galaxy. They focused on gas clouds, and revealed what they thought were 4 major star-forming arms: Norma, Scutum-Centaurus, Sagittarius and Perseus.

We live in minor arm called the Orion Arm, or the Orion Spur, located between the Sagittarius and Perseus Arms.

And then in 2005, astronomers used infrared telescopes to pierce through the clouds of gas and dust to see that the central bar in the middle of the Milky Way extends much further than previously believed.

In a new survey by Spitzer, astronomers merged together 800,000 photographs containing over 110 million stars. Software counted up the number of stars and measured their density.

As expected, astronomers found an increase in density in stars towards the Scutum-Centaurus Arm, but no increase towards the Sagittarius and Norma arms. The Perseus arm wraps around the outer portion of our galaxy and can’t be seen in the Spitzer images.

This helps make the case that the Milky Way only has two spiral arms; a commonly seen situation where a galaxy has a long central bar.

Original Source: NASA/JPL News Release

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

Sili June 3, 2008 at 12:59 PM

What do you wanna bet that this won’t shut up the people who claim that mass-extinctions are due to our passing through an arm?

tacitus June 3, 2008 at 1:01 PM

Does that mean that the Milky Way can now be reclassified as “Mostly Armless?”

Mek June 3, 2008 at 1:17 PM

Hahahaha @ tacitus :)

How will they reconcile the infra-red observations with the radio ones?

owen June 3, 2008 at 1:23 PM

very interesting!

Neil June 3, 2008 at 1:24 PM

definatly not funny!

Quantum_Flux June 3, 2008 at 2:41 PM

I suppose we can safely say that Vishnu didn’t create our galaxy in his image then because there are two few arms and legs. Nor did Jeebus for that matter because there are two few legs, and certainly our galaxy doesn’t even slightly resemble FSM unless that happens to be an elongated meatball in the center :)

Quantum_Flux June 3, 2008 at 2:42 PM

correction: Unless that happens to be an elongated meatball on a spinning fork in the center

tim June 3, 2008 at 6:55 PM

That news really bums me out, and at the same time peaks my interest at the idea of living in an entirely different looking galaxy than we’d been taught to believe. I guess my milkyway-galaxy, laser etched desk crystal is now a collectors item. I feel ripped off too, I want another. Lol.

Vagueofgodalming June 4, 2008 at 4:04 AM

It would be nice to have a diagram with the arms labelled, showing the old and new interpretations. JPL don’t seem to have published one.

LLDIAZ June 4, 2008 at 6:07 AM

I thought the theory was when we moved in and out of the galactic plain we were less protected not spiral arms.

GBendt June 4, 2008 at 1:04 AM

Since several years it is known that our Milky Way is a barred spiral galaxy. As barred spiral galaxies have two spiral arms, our home galaxy has two spiral arms, too.

NGC1365 gives a good impression of the look spectators from outside have on our Milky Way .

When the first maps of our galaxy were drawn some 60 years ago, the ability to interpret radio measurements was not that advanced than it is today, but these maps were a good to begin with.

Any idea someone has about our Milky way is necessarily based on the knowledge that he or she has. If some new findings increase the knowledge, someone´s idea of the Milky Way has to adapt respectively.

The last centuries of astronomical investigations came along with a permanent change of the contemporary idea of the structure our galaxy and the universe. This process of continuing change has not yet ended: I think it is really starting right now.

Regards,

Günther

alphonso richardson June 4, 2008 at 3:04 AM

Oh well………………

Jon June 4, 2008 at 5:06 AM

Hey Sili,

I’ll take that bet on mass-extinctions due to passing through spiral arms. Looking at the new picture of the Milky Way there sure seems to still be plenty of major and minor arms for the solar system to past through as it orbits around the center of the galaxy. Did you look at the new picture of the Milky Way at the begining of the article?

Although I’m not one of those people, I think this does not disprove their hypothesis. Care to elaborate on why you think otherwise?

Astrofreak June 4, 2008 at 7:32 AM

Hey Fraser, u turn over a new leaf? Ha! I didn’t find a single “might, could, may” et. al. in this story. I always heard we live nearby the “Orion Arm” but are not a part of it.

Aodhhan June 4, 2008 at 9:29 AM

This hasn’t put any old theories to bed. It will surely spark a whole mess of new ones.

neoguru June 4, 2008 at 10:55 AM

LLDIAZ is correct. The extinction theory involves passing in and out of the plane of the galaxy, not the arms.

Jon Hanford June 4, 2008 at 7:23 PM

Check out the annotated version of this pic at the Spitzer Space Telescope site. This makes a good T-shirt version of ‘Where am I ?’ (i.e. ‘Sun’)

El Sofista - La Vía Láctea sólo tiene dos brazos espirales June 5, 2008 at 12:45 PM

Si alguien nunca hubiera salido de su casa, no sabría cómo ésta se ve desde el exterior. Esa es la situación con la Vía Láctea. Estamos en su interior, así que en realidad no sabemos cuál es el aspecto de su estructura. Hay otros ejemplos de grandes espirales que podemos ver, pero eso es como ver otras casas desde la ventana de la nuestra y eso no permite asegurarnos de nada. [...] Fuente: Fraser Cain para Universe Today, y NASA/JPL.

Shabiq June 27, 2008 at 1:49 AM

Hey!! the astronomers are right, there are four arms(not exacty four,its more than four) not two because there are two star cluster in our galaxy, one in which new stars are formed and the other in which new stars are not formed.They have no gases, dust particles to form stars.So the density in stars towards the Scutum-Centaurus Arm increases, but do not increases towards the Sagittarius and Norma arms.

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