Life on The Edge

Now is a good time to buy real estate on Titan, the largest of Saturn's moons. Land there is dirt-cheap. But wait 5 billion years when the Sun begins its journey into retirement. As the Sun swells and becomes a red giant, life on Earth might get a little uncomfortable: The average temperature on our planet could catapult to a sizzling several thousand degrees Fahrenheit. Then it is time to reach for sunscreen with an S.P.F. of 2,000, or pack up your belongings and take the next space shuttle to a place with a more hospitable climate. That could be Titan, a moon larger than the planet Mercury, and about half the size of Earth. Titan is one of the safest bets to colonize because it is far enough from the Sun's death rattles, and it has an atmosphere to trap heat.

For those who find searing heat appealing, stick around. Earth will be the place for you. The weather will be fairly predictable. No snowstorms or ice storms, just extremely hot and dry. The only question is how large will the Sun get once it consumes its thermonuclear fuel - hydrogen - and begins expanding. Will the Sun swell so much that it engulfs Earth? Or will Earth just barely escape the Sun's grasp, only to be scorched by the dying star's prodigious increase in energy output as it fights off death? Scientists speculate about these two possible scenarios.

Sun Swallows Earth

Hot, bright, and foggy. This is the daily forecast if the Sun swallows Earth. Right now, Earth and the Sun are safely separated by about 93 million miles. But the Sun could reach 200 times its present radius during its expansion phase. Earth's atmosphere would quickly evaporate as the planet begins spiraling toward the Sun's core, which has heated up to 100 million degrees Fahrenheit. Of course, Earth would burn up before it reaches the core. Our planet's demise could take a few hundred to a few thousand years. But Earth would have company as it travels into the Sun. Other planets, such as Venus, would be swallowed up by the giant star.

Earth Barely Escapes the Sun's Grasp

Imagine a bloated, red Sun looming in the sky. Temperatures on Earth have catapulted to several thousand degrees Fahrenheit. This is life on the edge, when the Sun stops expanding just before reaching our planet.

Of course, barely missing getting swallowed is not much of a consolation. Earth's future still will be unpleasant. Either Earth will eventually evaporate or it will be subjected to a period of unbearable heat followed by an eon of extreme cold. The forecast will hinge on the Sun's ultimate distance from Earth. This distance will depend on how much mass the Sun loses as it swells during the expansion or red giant phase.

One possibility is that the Sun puffs up so much that it almost reaches Earth. Heat from this swelled star scorches our planet's atmosphere, vaporizes vegetation, and boils away its oceans. Earth looks like a wasteland. Because there is no atmosphere, the sky is black. The Sun is a huge, red orb that covers half the sky. Daylight is 3,000 times more intense than it is now. The intense heat eventually evaporates Earth.

Another theory is that the bloated Sun winds up far enough away from Earth that it does not burn off the atmosphere. This may sound like good news, but it is not. Earth's atmosphere acts like a greenhouse, trapping heat from the enlarged Sun.

Wasting Away

Regardless of Earth's fate, the Sun continues to wither away. A few thousand years after the Sun enters its twilight years, it peels off its outer layers, exposing its much hotter inner layers. Eventually the outer 40 percent of the Sun's mass will be puffed into space. Soon the Sun's carbon-oxygen core is uncovered. The core's surface temperature has risen to 250,000 degrees Fahrenheit, compared with a normal temperature of about 11,000 degrees Fahrenheit. The dense, hot carbon-oxygen star is not much larger than the Earth.

Once it retires as a white dwarf, the Sun has been reduced to a tiny, bright point of light. This hot cinder gradually cools off, sending Earth into a deep freeze. An icy rain - composed of material floating in Earth's sky - falls on our planet. After billions of years, the glowing cinder that was once our Sun burns out.

Won't Happen During a Human's Life Time

If a few opportunistic people decide to videotape these cataclysmic events, they will be very disappointed. The Sun's death cannot be recorded during a human's lifetime. Its journey into retirement will take more than a billion years. In fact, once the Sun begins to die in another 5 billion years, it will take about 1 billion years for the star to completely expand, and another 10,000 years for it to evolve from a planetary nebula to a fading white dwarf.

For the original page, link to Hubble Witnesses The Final Blaze of Glory of Sun-Like Stars
(News Release Number: STScI-1997-38 - Background at the HubbleSite)

Updated: December 23 '97

Best seen with MS Internet Explorer.

Back to: Hubble Witnesses The Final Blaze of Glory of Sun-Like Stars