All galaxies are going through some rate of star formation. New stars are being formed every year in the Milky Way. But some galaxies, classified as “starburst galaxies” are undergoing furious rates of star formation. Some are so active, they’re forming thousands of new stars every year.
So why do starburst galaxies form, when our own Milky Way has a relatively slow rate of new star formation? The most popular theory is that a galaxy is put into a starburst phase when it makes a close encounter with another galaxy. The gravitational interaction sends shockwaves through giant clouds of gas, causing them to collapse and form star forming regions. These create some of the most massive stars in the Universe; monster blue stars with more than 100 solar masses.
These massive stars live short lives and detonate as supernovae, blasting out more shockwaves into the galaxy. This creates a chain reaction that cascades through the galaxy. Within a few million years, the galaxy is forming stars at tens or even hundreds of times the rate of formation in a normal galaxy. And then when the gas is used up, within about 10 million years, the period of star formation ends.
Starburst galaxies are rare today, but astronomers have found that they were very common in the early Universe, when galaxies were closer and interacted more.
Thousands of starburst galaxies have been discovered across the Universe. One of the best known starburst galaxies is M82, located about 12 million light-years away in the constellation Ursa Major. The Hubble Space Telescope imaged the galaxy in 2005, and found 197 massive clusters of star formation going off simultaneously in the starburst core. The changes in M82 are being driven by its gravitationally interaction with nearby M81 galaxy.
We have written many articles about galaxies for Universe Today. Here’s an article about the starburst galaxy M82.
[/caption]
NBC news is reporting former astronaut Charles F. Bolden Jr. will meet with President Obama in the Oval Office on Monday morning and likely will be appointed the new NASA administrator. NASA has been without an administrator since January, and needs leadership as it faces big changes in the next few years, including the retirement of the space shuttle and the development of replacement vehicles to send humans to space. Bolden has flown four times to space, with more than 680 hours in Earth orbit. If appointed, he would be the first African-American administrator at NASA.
Bolden is regarded as a quiet man but not shy. He made his first spaceflight 23 years ago, and flew on the mission that deployed the Hubble Space Telescope.
Florida senator Bill Nelson flew in space with Bolden in 1986, just before the Challenger tragedy. “Charlie’s credentials are top-notch,” Nelson said. Former administrator Michael Griffin said Bolden would be “perfect” for the job.
A combination of radiation pressure from the star and the disk creates a net force that enables dust grains to surf along the disk surface from inner to outer regions of the disk. Credit: D. Vinkovic
Rocky planets like Earth are all believed to have begun as dust circling newly born stars, and clues about the origin of such dust comes to us in today’s meteorites and comets, as well as observations of circumstellar disks around young stars.
But mystery has shrouded the details of the evolution of dust and how it eventually comes to form larger objects. Now, two papers in the journal Nature are proposing a new mechanism to explain it.
The new mechanism hinges on heat-shocked crystalline dust grains, which somehow migrated from where they were created — presumably close to the Sun — to the outer Solar System. By implication, the same process should occur around other young stars.
A trio of past hypotheses had been proposed to explain the migration, but none of them quite fit. They included, according to physicist Dejan Vinkovic of the University of Split in Croatia, turbulent mixing, ballistic launching of particles in a dense wind created by interaction of the accretion disk with the young star’s magnetic field (called the X-wind model), and mixing mediated by transient spiral arms in marginally gravitationally unstable disks. Vinkovic is lead author on one of the Nature papers.
“The turbulent mixing requires a source of efficient turbulent viscosity and the magnetorotational instability is invoked as the most promising candidate, but large stretches of the disk are considered not sufficiently ionized to keep this instability active,” he wrote. “The X-wind model relies on the theoretical notion of magnetic field configurations in the immediate vicinity of pre-main-sequence stars and high hopes are put on future observations to resolve this predicament.”
And finally, “The spiral arms model is in the domain of discussions on whether the underlying numerics, physical approximations and assumptions about the initial conditions are realistic enough to make results plausible.”
In the other paper, Peter Abraham of the Hungarian Academy of Sciences and his colleagues find the signature of crystalline dust after a young star flared, whereas archival data showed no sign of it before the flare.
The Vinkovic paper investigates the mixing of large crystalline dust particles in the protoplanetary nebula around the young Sun.
The force produced by the light shining on an object is a well known phenomenon called radiation pressure. We do not feel it in daily lives because we are too massive for this effect to be noticeable. For very small particles, on the other hand, this force can be larger even than the gravity that keeps particles in the orbit around the star. Investigations have been focused so far only to the radiation pressure due to the starlight. The results showed that individual grains would not travel far and would be pushed deeper into the disk.
Vinkovic reports that infrared radiation arising from the dusty disk can loft grains bigger than one micrometer out of the inner disk, where they are pushed outwards by stellar radiation pressure while gliding above the disk. Grains re-enter the disk at radii where it is too cold to produce sufficient infrared radiation pressure support for a given grain size and solid density.
However, Vinkovic points out that it is not only the star, but also the disk that shines. When studying effects on protoplanetary dust grains larger than one micrometer, which is comparable to the particle size of cigarette smoke, Vinkovic has discovered that the intense infrared light from the hottest regions of the protoplanetary disk is capable of pushing such dust out of the disk. Infrared radiation is what we can feel as “heat” on our skin. Combination of radiation pressure from the star and the disk creates a net force that enables dust grains to surf along the disk surface from inner to outer regions of the disk.
The temperatures in this hot region reach around 1500 degrees Kelvin (2200 degrees Fahrenheit), enough to vaporize solid dust particles or to alter their physical and chemical structure. The mechanism that Vinkovic describes in his paper would transfer such altered dust particles to colder disk regions away from the star. This can explain why comets contain a puzzling combination of ices and particles altered at high temperatures. Astronomers have been perplexed by this mixture, since comets form in cold disk regions out of frozen substances like water, carbon dioxide or methane. Rocky dust particles that end up mixed with ices are therefore expected to never experience high temperatures.
In an editorial accompanying the studies, University of Missouri astrophysicist Aigen Li wrote that the origin of crystalline silicates in comets “has been a matter of debate since their first detection 20 years ago.”
While Li touts promise in the new theory, “It would be interesting to see whether other mechanisms such as turbulent mixing and the ‘X-wind’ model would effectively carry submicrometre grains, which are efficient mid-IR emitters, outwards and incorporate them into comets,” he wrote. “It is also possible that some — but not all — crystalline silicates are made in situ in cometary comae.”
Source: Vinkovic’s press release. Watch a short animation showing how the newly proposed mechanism of dust movement works.
[/caption]
A typical spiral galaxy is shaped like a flat spinning disk – think of a record. It has a bulging galactic core surrounded by a flat rotating disk of stars. For example, our own Milky Way measures about 100,000 light-years across. Our Sun is thought to be about 25,000 light-years away from the galaxy core.
Studying the galaxy core is very difficult for astronomers. That’s because the regions surrounding the central core are shrouded in thick gas and dust that blocks visible light. In order to study the center of the galaxy, astronomers used to have to look at other galaxies that were similar in structure to the Milky Way. But in the last few decades, astronomers have been finally able to study the galaxy core in other wavelengths, like infrared and x-rays, which can pass through gas and dust.
And what they found surprised them.
Researchers discovered that the stars at the galactic core are orbiting an object with an enormous amount of mass. That object turned out to be a supermassive black hole, with 4.1 million times the mass of the Sun. Since that discovery, astronomers have located supermassive black holes in the galactic cores of many galaxies, and theorized that they’re in all galaxies.
Active galaxies, known as quasars (as well as other names), occur when the supermassive black hole is actively feeding on infalling material. This material heats up to millions of degrees and blazes with more radiation than all of the stars in the galaxy. And then when the supermassive black hole at the galaxy core runs out of fuel, it goes quiet again.
Within a parsec of the galactic core, there are thousands of stars. Most of these are old main sequence stars, there are many massive stars too. In fact, more than 100 of the brightest, hottest types of stars have been discovered around the galaxy core. Astronomers used to think that massive tidal forces from the supermassive black hole at the center of the galaxy would prevent their formation, but there they are.
We have written many articles about galaxies for Universe Today. Here’s an article about how a collision between galaxies creates a dark matter core.
Traveling to distant locations, like Andromeda, could have interesting consequences.
Credit: NASA
[/caption]
Andromeda, M31, Triangulum, NGC 2403 the Whirlpool… have you ever wondered how galaxies get their names?
Galaxies usually have several names. That’s because there are several catalogs that maintain the names. For example, there’s the Messier catalog of objects. This was a list of 110 fuzzy objects that Charles Messier maintained that could be confused with comets.
There’s another list that starts with NGC. For example, NGC 7331, a galaxy that has been called a twin of the Milky Way because of its similarity. The NGC catalog is short for New General Catalogue, and it’s a list of 7,840 interesting objects in the night sky.
So let’s take a look at an object like Andromeda. It’s named the Andromeda Galaxy because it’s located in the constellation of Andromeda. Many galaxies are named after the constellation they’re located in. Andromeda also has the designation M31, or Messier 31, since it’s the 31st object on Messier’s list of things that look like comets but aren’t comets. Andromeda is also designated as NGC 224 in the New General Catalogue.
There are also specialty catalogs that describe objects in other wavelengths, like x-ray and even gamma rays. And many galaxies will have “names” in those directories as well.
So a galaxy can have many names. It just depends on which name you want to use.
If you discover a galaxy, do you get to name it? Unfortunately, no. The official names for astronomical objects are maintained by the International Astronomical Union. Just how you can’t officially name a star after yourself, you can’t name a galaxy either.
We have written many articles about galaxies for Universe Today. Here’s a more information about naming a star.
[/caption]
Just like the Earth orbits the Sun, the Sun itself is part of the Milky Way galaxy. It takes about 220 million years for the Sun to complete a single journey around the Milky Way. But the Sun also bobs up and down as it travels in orbit around the center of the galaxy. The oscillation takes a total of 64 million years to complete. And there’s a moment when the Sun passes directly through the galactic disk and there’s a perfect galactic alignment between the Sun and the center of the galaxy.
When’s that galactic alignment going to happen? It’s almost impossible to know exactly. The Milky Way is 100,000 light-years across, but only 1,000 light-years thick. So during the course of that 64 million year cycle, the Sun rises above the galactic plane 500 light-years, passes down through the galactic plane, until it’s 500 light-years below and then comes back up again.
There has to be a moment when everything’s in perfect alignment, but the timescales are so long that astronomers couldn’t calculate it. Of course, this alignment with the center of the galaxy doesn’t have an effect on the Earth or the Solar System, it’s just like crossing an imaginary line in space, like traveling from Canada to the United States in your car.
There’s another type of galactic alignment. This is where the Earth, Sun and the center of the galaxy are in perfect alignment from our perspective. This actually happens every year during the winter solstice, on December 21st. Because of a wobble in the Earth’s orbit, the positions of the constellations slowly shift from year to year. The most perfect galactic alignment between the Earth, Sun and the center of the Milky Way happened back in 1998, but now we’re slowly shifting away from that alignment. In the coming decades, the perfect alignment will shift to another day.
Again, the alignment of these objects is purely a coincidence.
We have written many articles about galaxies for Universe Today. And we’ve written many articles about the 2012 myth. Here’s even more info on the supposed 2012 planet alignment.
[/caption]
The Milky Way is a spiral galaxy, measuring 100,000 light-years across, but only 1,000 light-years thick. Imagine a spinning record, but made of stars. The galactic equator is the halfway point between the top and bottom of that disk. Imagine you were traveling in a spaceship through the galactic plane. The moment you cross the midpoint of the disk, that’s the galactic equator.
Astronomers measure the position of an object in the sky using a coordinate system that’s sort of like latitude and longitude on Earth. They use two numbers: right ascention and declination. Right ascention gives a position east/west in the sky, and declination measures north/south. But there’s another measurement system that some astronomers use called the galactic coordinate system, which uses the galactic equator of the Milky Way.
This system imagines the Sun at the center, with 0-degrees being a line drawn straight from the Sun to the center of the Milky Way. You can then measure locations in a circle around the galactic equator. You can also measure locations above and below the galactic equator.
The north galactic pole is perpendicular to the galactic equator – 90-degrees above the equator, and the south galactic pole is below.
You might be interesting to know that the Solar System bobs up and down above and below the galactic equator. It takes 64 million years to complete a full cycle going above and below the galactic equator. If you’re heard that the Solar System is supposed to cross the galactic equator in 2012, don’t worry, that’s a myth. It takes 64 million years to complete that cycle, so there’s no way to know exactly when it will actually cross the galactic equator.
[/caption]
The Milky Way is a vast spiral galaxy, shaped a bit like a spinning record; just one that measures 100,000 light-years across and only 1,000 light-years thick. Imagine you were below the Milky Way, and passed through the disk of stars above it. That moment when you’re halfway through the 1,000 light-year thickness of stars? That’s the galactic plane.
Astronomers actually use a coordinate system to measure positions in the Milky Way, starting with the Sun as the center point. No, we’re not actually at the center of the Milky Way, we’re actually off to the side, but this makes the measurement easier. They draw a line from the Sun to the center of the Milky Way, and that defines the 0-degree point, and then coordinates are measured within the galactic plane. You can have galactic latitude and longitude.
Have you heard anyone mention that the Sun is supposed to be crossing the galactic plane in 2012? Yeah, that’s a myth. Here’s the thing. The Sun does bob up and down in the galactic plane. Sometimes we’re above the plane, and then other times we’re below the plane. But that cycle takes 64 million years to complete! It’s impossible to define the exact moment of when the Solar System will pass exactly through the galactic plane.
And astronomers don’t think that anything special will happen when the Solar System does pass through the galactic plane. In fact, it’s the times when the Earth is above or below the galactic plane when we might be at risk. A recent scientific study correlated those times with large extinction events in the Earth’s history. It’s possible that the Milky Way’s magnetic field protects the Earth from intergalactic radiation and cosmic rays, and when we’re significantly above or below the galactic plane, life on Earth suffers more damage from space radiation.
But that’s just a theory.
So, to summarize, the Solar System won’t be passing through the galactic plane in 2012. There’s no easy way to know exactly when that’ll happen, and there’s absolutely no way to give that a specific date. And even when we do pass through the galactic plane, there’s no risk to our planet.
Traveling to distant locations, like Andromeda, could have interesting consequences.
Credit: NASA
[/caption]
The distance to the Andromeda Galaxy is 2.54 million light-years, or 778 kiloparsecs.
The Andromeda Galaxy can be seen with the unaided eye, so skywatchers have been observing it for thousands of years. Charles Messier cataloged it as M31 in his 1764 list. Back then, astronomers thought that Andromeda was a nebula, and based on its size, Messier guessed that it was only about 2,000 times further than the star Sirius.
Astronomers discovered variable star called novae in Andromeda in 1917, and quickly realized that they were 10 times less bright than similar objects in the Milky Way. Astronomers Heber Curtis proposed that Andromeda was a separate “island universe”, located about 500,000 light-years away. Edwin Hubble ended the controversy once and for all in 1925 when he identified Cepheid variable stars in Andromeda, and calculated that the galaxy was actually 1.5 million light-years away.
Modern astronomers are continuing to calculate the distance to Andromeda. In 2003, astronomers calculated that Andromeda is 2.57 million light-years away. And in 2004, astronomers redid Hubble’s Cepheid variable calculations, and determined that Andromeda was 2.51 million light-years. Another group used a different technique in 2005 to calculate that Andromeda was 2.52 million light-years away. And yet another technique in 2005 put it at 2.56 million light-years away. And so, the agreed distance of 2.54 million light-years is an average of the distances measured so far.
There are several galaxies closer to Earth than Andromeda. The Large Magellanic Cloud is only 160,000 light years away, and the Canis Major Dwarf Galaxy is a mere 25,000 light-years from Earth. But Andromeda is the largest grand spiral galaxy to us.
We have written many articles about galaxies for Universe Today. Here’s another article about the closest galaxies to the Milky Way.
Artist's Conception of our Milky Way Galaxy: Blue, green dots indicate distance measurements. CREDIT: Robert Hurt, IPAC; Mark Reid, CfA, NRAO/AUI/NSF
[/caption]
Look across the Universe, and you’ll see that almost everything is rotating. The Earth rotates on its axis as it orbits the Sun. And the Sun itself is rotating. As you can probably guess, we even have galaxy rotation with our Milky Way galaxy.
Our galaxy is rotating incredibly slowly, however. It takes the Sun 220 million years to complete a single orbit around the galaxy. In the 4.6 billion years that the Sun and planets have been here, they’ve only rotated around the center of the galaxy about 20 times.
We know that galaxy rotation is happening because the Milky Way is a flattened disk, in the same way that the Solar System is a flattened disk. The centrifugal force from the rotation flattens out the galactic disk. All stars in the galactic disk follow roughly circular orbits around the center of the galaxy. Stars in the halo can have much different orbits and speeds.
The calculation of the high rotational speed of the galaxy led to the discovery of dark matter. If our galaxy contained just the matter we can see – planets, gas, etc – the galaxy rotation should cause it to spin apart. Instead, there’s much more mass holding the galaxy together. In fact, astronomers have calculated that the total mass of the galaxy is probably 10 times greater than the sum of all the stars in it. 90% of this is invisible dark matter, holding the galaxy rotation together. And only 10% is the regular matter that we can see. Our galaxy really has a mass of more than 1 trillion suns, and extends out more than 600,000 light-years; a third of the distance to the nearby Andromeda galaxy.
All the galaxies we can see are rotating. It’s this rotational force that counteracts the inward pull of gravity from all the galaxies. If galaxies didn’t rotate, they’d collapse inward and just join the supermassive black holes at the hearts of galaxies.
We have written many articles about galaxies for Universe Today. Here’s another article about the rotation of the Milky Way.
