The computer-graphics movie was spellbinding. Members of the Salt Lake Astronomical Society watched intently as regions of denser material swirled, condensed and slid around, representing events inside a vast molecular cloud in space happening over a period of 250,000 years.
“Notice it’s a very random kind of process,” said the speaker, Wayne Sumner. “Things crash together, things fly apart. It’s not linear.”
[Wayne Sumner discusses star formation, April 19 at the monthly meeting of the Salt Lake Astronomical Society. Photo by Cory Bauman. For more information about SLAS, including free star parties and how to join the group, click HERE.]
A math and science teacher at Northridge High School, Layton, and an adjunct professor at Weber State University, Layton, Sumner was describing the formation of stars within what are termed giant molecular clouds. He spoke during the society’s monthly meeting, April 19 at the Engineering/Mines Classroom Building, University of Utah.
GMCs occur throughout galaxies and are basic to the creation of stars. The clouds, made up of gas molecules and dust, can vary from a few light-years to hundreds of light-years across. To give an idea of the scale, he said, “Bear in mind the closest star to us [besides the sun] is only 4.3 light-years away.”
They are called clouds because they are “fluffy, kind of nebulous things.” They are somewhat opaque, and energetic particles don’t penetrate easily to break up the first stages of clumping. That means “the granules of things inside them can stick together to form molecules.”
How rarefied are they? Sumner said the amount of air molecules between the eye of a seated person and the floor – approximately 1 cm. by 1 cm. by 1 meter (0.6 inch by 0.6 inch by 39 inches) – is equal to the number of molecules in a stretch of a GMC that is 1 cm. by 1 cm. by 25,000 light years. A good vacuum on Earth would be one-billionth as rarefied, he said.
In addition, the clouds are extremely cold, with many parts only 10 Kelvin (which is minus 441.4 degrees F). Yet they glow and some are visible to backyard telescopes.
“Why is it so luminous? Because it’s humongous,” he said. The brightness of an object depends on this formula: luminosity equals the energy emitted per square yard times the overall area. An object that puts out only a little light per square mile can appear bright through a telescope if the object covers enough square miles.
“For reasons that we really don’t understand very well, sometimes certain regions within this giant molecular cloud will start to gravitationally attract” material. These regions, called cores, form where parts of a GMC are dense enough. They continue to accumulate swirling gas and dust, becoming yet denser as their gravity pulls in more material.
But what causes the gas to become dense enough to form cores in the first place? “In fact, this is an area of much research and disagreement right now,” Sumner said. The main candidate for triggering such chances in the cloud is turbulence, he said. Turbulence could arise from shockwaves from nearby supernovas or pressure changes from stars in the region. The GMC’s gas is chaotic, so its actions are hard to understand. “We’re not very good at predicting weather yet,” let alone such exotic phenomena.
“What starts this process is not very well known. But we’re working on it.”
Galaxy collisions are known to create stars in GMCs.
“When galaxies collide, stars never hit because they’re separated by light years. But these giant molecular clouds do.” When molecular clouds in Galaxy A rip through those of Galaxy B, the collisions can trigger a round of star formation. Density waves that move within spiral galaxies are another cause of turbulence.
Sumner said temperature, pressure, and angular momentum slow down the star’s formation. In the end, gravity wins over forces like these that otherwise would blow away material. When enough matter has accumulated in the core, gravity makes it suddenly collapse into a protostar.
A remnant of the larger core surrounds the protostar, and as the protostar continues to gather material and shrink, it becomes hotter. “These protostars collapse from the inside out …. The stuff at the center collapse first and then the stuff at the top collapses on top,” he said.
As this is going on, gravity increases, the protostar becomes denser, and the collapse speeds up. The protostar heats to the point that its hydrogen molecules begin to fuse into helium, releasing enormous amounts of energy, and a star shines out of the dust.
[NASA’s Spitzer Space Telescope made this view (cropped here) of a star 100,000 times as bright as the sun shining within a giant molecular cloud 6,200 light-years away in the constellation Cygnus. The cloud’s dust hid the star until the image was taken through Spitzer’s specialized filters. Photo released April 13, 2004. Credit: NASA/JPL-Caltech]
The dusty disk around the newborn star may clump together and form planets. Eventually the star’s solar wind blows much of the surrounding material away.
What determines the size of a star? “I’ll tell you: we don’t know but we’re working on it,” Sumner said. “Are the processes of formation of large stars and small stars the same process? Some say yes and some say no.”
At talk’s end he again projected the video with its swirling gas and material moving toward and away from denser parts. Look at the chaotic action, Sumner said, “and then contemplate how we all came to be here.
“It’s astonishing to me.”