How Do Galaxies Grow?

How Do Galaxies Grow? Massive Galaxies Caught In The Act Of Merging

ScienceDaily (Aug. 26, 2008) — Astronomers have caught multiple massive galaxies in the act of merging about 4 billion years ago. This discovery, made possible by combining the power of the best ground- and space-based telescopes, uniquely supports the favoured theory of how galaxies form.

How do galaxies form? The most widely accepted answer to this fundamental question is the model of 'hierarchical formation', a step-wise process in which small galaxies merge to build larger ones. One can think of the galaxies forming in a similar way to how streams merge to form rivers, and how these rivers, in turn, merge to form an even larger river. This theoretical model predicts that massive galaxies grow through many merging events in their lifetime. But when did their cosmological growth spurts finish? When did the most massive galaxies get most of their mass?

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NASA Images Show Gamma Ray Bursts Across Milky Way

This all-sky view from Fermi reveals bright gamma ray emissions in the plane of the Milky Way, center, and in bright pulsars and super-massive black holes. (Nasa/u.s. Department Of Energy/international Large Array Telescope Team)

By Sindya N. Bhanoo

Washington Post Staff Writer
Wednesday, August 27, 2008; Page A03

NASA researchers yesterday released images collected by a new telescope studying high-energy gamma rays. A combined image from 95 hours of the telescope's initial observations showed bursts of gamma rays glowing across the plane of the Milky Way.

The Gamma-Ray Large Area Space Telescope, renamed Fermi, was launched in June and is off to a promising start, NASA scientists said.

"I like to call it our extreme machine," said Jon Morse, the director of astrophysics for NASA. "It will help us crack the mysteries of these enormously powerful emissions."

Gamma rays are powerful light rays invisible to the naked eye. Because Earth's atmosphere absorbs gamma rays, they can be studied only from the outskirts of the universe.

Fermi is gathering data on gamma rays that originate near black holes and high-energy stars called pulsars.

Though much remains unknown, bursts of gamma rays are thought to be emitted from particles coming out of black holes and pulsars, said Peter Michelson, a Stanford physicist and a principal investigator for the mission.

"We don't yet understand the mechanism for how the particles that emit the gamma rays are accelerated," Michelson said. "We're not even sure what the nature of the particles are."

The study is a follow-up on work done by the Energetic Gamma-Ray Experiment Telescope, a mission that studied gamma rays from 1991 to 2000.

Fermi's technology allowed scientists to compile in days what took the first mission one year to do, said Steve Ritz, one of the project's scientists.

The telescope was renamed Fermi yesterday, after Italian physicist Enrico Fermi, because he is "today regarded as one of the top scientists of the 20th century," Ritz said.

The scientists hope that in the five to 10 years that it is in orbit, Fermi will be as remarkable as its namesake.

"This powerful space observatory will explore the most extreme of environments for us," Morse said.

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Solar System Is Dented, Not Round: need to change the solar system models

Space Probes Show Solar System Dented, Not Round
Astronomers say they'll have to change their solar system models
By SETH BORENSTEIN AP Science Writer
WASHINGTON July 2, 2008 (AP)
The Associated Press


When viewed from the rest of the galaxy, the edge of our solar system appears slightly dented as if a giant hand is pushing one edge of it inward, far-traveling NASA probes reveal.

Information from Earth's first space probes to hit the thick edge of the solar system — called the heliosheath where the solar wind slows abruptly — paint a picture that is not the simple circle that astronomers long thought, according to several studies published Thursday in the journal Nature. Surprised astronomers said they will have to change their models for what the solar system looks like.

In 1977, NASA launched two space probes on missions beyond the solar system. Voyager 1 went north and Voyager 2 went south. What startled astronomers is that when the two of them hit the heliosheath they did so at different distances from the sun.

Voyager 2 hit the southern edge of the solar system nearly 1 billion miles closer to the sun than Voyager 1 did to the north. Voyager 2 hit the edge at 7.8 billion miles from the sun.

"We used to assume that it's all symmetric and simple," said Leonard Burlaga, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md. "It's literally like a hand pushing."

That push is from the magnetic field that lies between star systems in the Milky Way. The magnetic field hits the solar system at a different angle on the south than on the north, probably because of interstellar turbulence from star explosions, said Voyager project scientist Ed Stone.

Both spacecraft still have several more years before they completely exit the solar system and continue deeper into the space between stars, said Stone, former director of NASA's Jet Propulsion Lab.

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New Supernova

Discovery of the Week
The Hot New Supernova



Stellar scientists announced last week that they've spotted the youngest supernova remnant ever seen in the Milky Way. Called "G1.9+0.3," the remnant's estimated age is just 140 years--meaning that's when the radio waves started reaching Earth. (The actual supernova happened 26,000 years ago.)

How young is that, in supernova remnant terms? Until last week, the youngest remnant ever seen in our galaxy was a 330-year-old named "Cassiopeia A." So, why hadn't scientists spotted "G1.9+0.3" before, if its radio waves first reached us in the 1860s? Because to see it, they had to peer through a thick cloak of interstellar dust.

While astronomers pore over their super new data, we're here to help you understand what supernovas really are. It's all about the hot life and violent death of a big star.



A Star Is Born

Like humans, stars are born through contractions--though the contractions here are not of muscle, but of massive clouds of gas and dust in interstellar space. Every now and then, such a cloud accumulates enough matter for gravitational forces to pull it together even more. A protostar is born, and gets hot. When the temperature near its center hits 18 million degrees Fahrenheit (10 million degrees Celsius), nuclear reactions kick in.

Newborn stars are made mostly of hydrogen. At their cores, they "burn" hydrogen and generate helium. Of course, they don't use matches or flames. The burning at a young star's heart is a nuclear fusion reaction, in which four hydrogen atoms fuse to produce a single helium atom. The mass of that helium atom is less than the combined mass of the four hydrogen atoms, and the leftover mass is released as energy.

The release of that energy drives the temperature inside the star way up--in some cases to hundreds of millions (even billions) of degrees Fahrenheit. Pressure inside the star increases enough to counteract the gravitational forces still trying to contract it. At the same time, heat pours from the star's core toward its cooler surface, and from there radiates into space. Presto: a relatively stable star is burning bright.

Twinkle, Twinkle, Supergiant Star

Stars survive a long time by human measures, but eventually they all run out of gas, literally. And those that live larger burn out quicker. A relatively small star--like our sun--might burn for 10 billion years, and then linger for eons as a cosmic cinder called a "white dwarf." A star 10 times as massive might live just 10 million years, and then go out with a bang.

When a star's core runs out of hydrogen to burn, it begins to contract again. The core's temperature increases until the helium made earlier ignites. Now a helium fusion reaction produces carbon and oxygen in the star's core, while hydrogen fusion fires up in a thin shell around it. The star generates far more energy than before, and puffs up accordingly. If the star started out modest, it grows into a red giant. If it started out big, it becomes a supergiant.

Smaller stars' nuclear careers generally end with the burning of core helium. But big stars start burning the carbon and oxygen fused in the helium reaction, too. They go on to produce elements like neon, magnesium, silicon, and sulfur. Then they burn the silicon to produce iron. Such stars wind up layered like onions--with a central core of iron, surrounded by layers of burning silicon, magnesium, neon, oxygen, carbon, helium, and hydrogen.

Out with a Bang

After taking several million years to grow up, a supergiant builds its iron core in about a day. At its peak, the iron core is around two-thirds the size of the Earth but contains more mass than the sun. It's also caught in an enormous gravitational crunch. The star's core no longer generates energy to counteract the forces of contraction--to fuse iron requires energy input rather than leading to energy release--so it can't hold out for long.

When it goes, it goes fast. In less than a second, the core collapses from a 5,000-mile-wide sphere (8,000 km) into a 12-mile-wide one (20 km). The sudden crash releases a huge amount of energy--100 times the energy our sun will produce in its entire 10-billion-year life. Tiny particles called neutrinos carry most of that energy off into space. The rest races out through the star's layers in a supercharged shockwave.

The resulting explosion blasts the star's gaseous shell into space at speeds exceeding 10 million miles per hour (16 million km/h). For a few weeks, this "supernova" burns brighter than a billion suns. And for millennia to come, the former star's gaseous shell plows into the interstellar medium. Meanwhile, the star's collapsed iron core carries on as a neutron star--or, in some cases, becomes a black hole. In the cosmos, it seems, big stars burn out and fade away.

--Steve Sampson