StarDate

The Sun belongs to a rare class of stars: Class G. Members of this class account for only seven or eight percent of all the stars in the Milky Way Galaxy.

A star’s class is determined by its surface temperature, which we see as its color. G stars are in the middle of the temperature scale, so they shine yellow or yellow-white.

Most class-G stars are in the prime of life – a span that puts them on the main sequence. They’re steadily “fusing” the hydrogen in their cores to make helium. And most of them will stay in that phase of life for 10 billion years or longer.

Most of them are within about 15 percent of the mass of the Sun. Any heavier and they’d get hot enough to move up to class F or higher. Any lighter and they’d be cool enough to move down to class K or M.

Main-sequence G stars are only a small fraction of the size, mass, and brightness of the top stars. But the stars in higher classes are even more rare than G stars. So the Sun, which is near the top of its class, shines brighter than about 90 percent of the stars in the Milky Way.

Not all G stars are on the main sequence, though. Some are bloated and heavy. Such stars are more massive than the Sun. They’ve burned through the hydrogen in their cores, and are fusing hydrogen in a thin shell around the core. But they’re undergoing a transition, so they won’t stay yellow for long – the short-lived monsters of class G.

Script by Damond Benningfield

Under a dark sky, far from city lights, the eye alone can see thousands of stars. They come in a variety of brightnesses and colors. Yet only a few of those pinpoints are like the Sun.

Stars are classified based on their surface temperature, which is indicated by their color. Hot stars are blue and white, while cool stars are orange and red. The Sun is in the middle, shining yellow-white, with a surface temperature of about 10,000 degrees Fahrenheit. Those traits mean it’s class “G.”

From hottest to coolest, stars are classified with the letters O, B, A, F, G, K, and M. Each class is divided into 10 sub-categories. And each star is given a Roman numeral that indicates its size and its stage in life.

The Sun is class G2V. That means it’s toward the hot end of class G, and that it’s on the “main sequence” – it’s in the prime of life, converting the hydrogen in its core to helium.

A couple of moderately bright G main-sequence stars are in view by late evening. 51 Pegasi is high in the southeast, while Tau Ceti is just climbing into view far below it, close to the horizon.

The brightest class G main-sequence star visible from Earth is Alpha Centauri A. It’s the leading light of a triple star system that’s a bit more than four light-years away – closer than any other stars. It’s so far south, though, that it’s visible from only a tiny sliver of the United States.

More about G stars tomorrow.

Script by Damond Benningfield

To borrow from an advertising campaign, Jupiter is “getting the red out.” Its enormous “eye” – the Great Red Spot – has been shrinking. It’s now just a third as wide as when it was first seen, and it’s getting smaller all the time.

The Great Red Spot is one of the most recognizable features in the solar system. But scientists still don’t know that much about it. They don’t know for sure how it formed, why it’s shrinking, or even why it’s red. They have lots of ideas, but no certain answers.

The spot was discovered as early as 1831. There were suggestions that it was seen much earlier, but those sightings might have been a different feature. The spot has been tracked on a regular basis since 1878. It’s between two powerful jet streams. They prevent it from wandering to different latitudes. It tops out several miles above the surrounding clouds, and extends at least 150 miles below the clouds.

In the late 19th century, the spot was a long oval – about as tall as Earth, but three times as wide. Today, it’s roughly the same size as Earth. And the rate of shrinkage has been increasing. So it’s possible that it could disappear entirely within a few decades.

Look for Jupiter close below the Moon at dawn tomorrow. It looks like a brilliant star. The Great Red Spot is visible through a telescope, but whether it’s visible from a particular location depends on the timing.

Script by Damond Benningfield

The Moon anchors a prominent triangle in tomorrow’s early-morning sky. The pattern is well up in the east at first light. The stars Pollux and Castor – the twins of Gemini – line up to the upper left of the Moon, with Mars to the upper right. The brilliant planet Jupiter stands well below the triangle.

Mars and Pollux are almost exactly the same brightness right now. And they’re the same color: orange. But they achieve that color in different ways.

Mars is a planet – a ball of rock and metal that’s smaller than Earth. Its color comes from iron oxide – particles of rust – in the rocks and dust that cover most of its surface. The rust probably formed when iron-rich rocks interacted with liquid water on the surface. But there’s no water on the surface today. So the rocks must have rusted billions of years ago, when Mars was much warmer and wetter than it is today. As the rocks eroded, the Martian winds carried the dust around the globe – enhancing the color of the Red Planet.

Pollux, on the other hand, is a star. It completed the prime phase of life, then puffed up to giant proportions – about nine times wider than the Sun. As it expanded, it got cooler. And a star’s color is determined by its surface temperature; cool stars look red or orange. So just by looking at it, we can tell that Pollux is thousands of degrees cooler than the Sun.

We’ll talk about the Moon and Jupiter tomorrow.

Script by Damond Benningfield

Some of the most imposing features on Mars are its giant volcanoes. The largest is Olympus Mons. It’s more than 13 miles high, and covers an area as big as New Mexico. It’s part of the largest complex of volcanoes on the planet – a region called Tharsis Ridge.

The second-largest group is on Elysium Rise. Its largest member is Elysium Mons. It’s the fourth-highest mountain on the planet. It has an elevation of about 10 miles above the Martian equivalent of “sea level,” and it towers about eight miles above the surrounding plains.

Like the other major volcanoes, it’s extinct – or at least dormant. It probably hasn’t erupted in hundreds of millions of years. It formed over billions of years, from layers of lava and ash. It’s marred by many craters.

Some of them are impact craters, carved by giant space rocks. Others may be volcanic vents, formed by side eruptions of gas or lava.

The volcanoes on Mars have grown so big mainly because there are no plate tectonics. Once a pool of magma forces its way to the surface, it just keeps going – the crust above it doesn’t move away. So there’s no “cut-off” valve – the volcano erupts as long as there’s molten rock below to keep feeding it – building some giant mountains on the Red Planet.

Mars appears below the Moon in tomorrow’s early morning sky. It looks like a fairly bright orange star.

More about the Moon and its companions tomorrow.

Script by Damond Benningfield