Sky Lights

In the heavens as on Earth, reliability is rare, precious, and well, unreliable

By Bob Berman|Thursday, March 31, 2005

The 20th of this month marks the vernal equinox, a day of reassuring constancy. At this time each year, everyone in the world sees the sun rise due east, provide 12 hours of daylight, then set due west to usher in 12 hours of night.

The more closely astronomers examine the sky, however, the more they see not constancy but variation. Sunlight is unsteady, it turns out, and starlight is unsteady too. The glow from remote galaxies abounds with additional forms of variability. Now researchers are struggling to quantify the changes and to uncover meaningful patterns within them. Their answers may reveal the future of our planet and even the destiny of the universe.

Take the sun. Satellite measurements show that its seemingly steady brightness rises and falls over an 11-year cycle. The change is small—about 0.1 percent—but other, longer-term shifts in solar output seem to correlate with noticeable shifts in earthly climate. By far the worst is yet to come. The sun is becoming increasingly brilliant as it ages. In 1.1 billion years it will be 10 percent brighter than it is today, enough to boil off the oceans and prompt a run on Martian real estate.

Other stars are even less steady. Betelgeuse, the prominent red star marking Orion’s shoulder, sometimes outshines its blue neighbor, Rigel, but other times lags well behind. It is a bloated giant whose extended atmosphere expands and contracts over a semiregular cycle of about five years. Mira, a similar but more extreme star in the constellation Cetus (low in the west at nightfall this month), can change in brightness by a factor of 1,500.

Even the iconic North Star, Polaris, is unreliable. Once considered the standard of brightness, it turns out to be in a class of stars called Cepheid variables. These stars pulsate like Mira and Betelgeuse but follow a much more regular pattern. A century ago, Polaris’s light flickered by about 12 percent on a four-day cycle. For reasons unknown, its fluctuations are dying out, making it unlike other Cepheids. Polaris is inconstant even in its inconstancy!

Yet there is a hidden order inside some of this cosmic capriciousness. Henrietta Swan Leavitt discovered in 1912 that the period of variation among Cepheids correlates with how much light they emit—their luminosity. The longer the period, she found, the more luminous the star. Once you determine a Cepheid’s period, you know its inherent brightness. You can then measure its apparent brightness and determine how far away it is. A dozen years later, Edwin Hubble used this relationship to measure the distances to other galaxies for the first time.

Cepheids are not bright enough to be seen beyond our local galaxies, so astronomers have sought more prominent reference lights. In the 1990s, they recognized that a class of exploding stars called Type Ia supernovas might fit the bill. These titanic blasts are caused by collapsed stars that snatch matter from their companions until they become so massive that they detonate. Type Ia’s follow a consistent pattern that can reveal their true luminosity.

Through a careful monitoring of these supernovas, two research groups were able to determine the distances to galaxies billions of light-years from Earth. This work led to the discovery that the expansion of the universe seems to be growing faster, not slower, over time.

Last year astronomers discovered evidence of another unexpectedly uniform kind of variability among gamma-ray bursts, stellar explosions that are even more luminous than Type Ia supernovas. These mysterious objects can be clearly seen all the way across the visible universe. Equally important, some researchers argue that the pattern of energy emitted by each burst may be linked to its luminosity. If so, gamma-ray bursts could help astronomers map the most distant reaches of the cosmos. Four months ago, NASA launched the Swift satellite to watch for gamma-ray bursts. First results are coming in now.

The message from all these Cepheids, supernovas, and bursts sounds almost paradoxical. Complete constancy does not solve cosmological puzzles, and neither does inconstant variability. Instead, astronomers seek predictable inconstancy. It’s enough to make one appreciate the simple and reliable old equinox.

What’s up in March
March 1-15: Mercury is at its best and brightest of the year, easily seen in evening twilight. The moon is near it on the 11th.

March 3: The last-quarter moon passes in front of the bright star Antares, a pretty coupling for those willing to get up before dawn.

March 20: The sun is directly over the equator at 7:33 a.m. EST, marking the official beginning of spring.

March 25: Jupiter forms a close pair with the full moon; both objects rise at nightfall.

March 25: 350th anniversary of Christian Huygens’s discovery of Saturn’s largest moon, Titan.

All month: Daylight grows at its fastest rate of the year; four minutes daily in the northern states, one to two minutes per day in the south.

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