Over the next few days you can watch the planet Mars slip very close by a bright star in the constellation Gemini. The star has several names: some call it Epsilon Geminorum, because it is the (alpha, beta, gamma, delta, epsilon, …) fifth brightest star in the constellation Gemini; others more familiarly call it Mebsuta. But by any name, the star tells an interesting story.
Look for Mars and Mebsuta in Gemini high overhead an hour or so after sunset. Sunday night, at their closest approach, the pair will shine only one-third of a degree apart. A degree of arc is about the width of your index finger held at arm’s length.
Mars slides through Gemini, just past the star Mebusta (aka Epsilon Geminorum) in late March/early May.
Mebsuta is, of course, much further away than Mars. In fact, at a distance of about 900 light years, Mebsuta is one of the more distant stars we see in the night sky. Yet Mebsuta is pretty bright, making the list of the top 200 brightest stars.
One way a distant star can appear very bright is if it is very hot. All other things equal, a hotter star is brighter than a cooler star. But this is not the case for Mebsuta; its surface temperature is only about 4,500 degrees, considerably cooler than the sun. Mebsuta is bright because it is very large. And this is where Mebsuta’s story gets interesting.
Mebsuta lies near the ecliptic, the imaginary line around the sky that the sun, planets and moon never stray too far from. The ecliptic maps out the plane of our solar system. Because Mebsuta lies near the ecliptic, solar system bodies, including the moon and planets, sometimes pass in front of it. Astronomers call this an occultation.
Astronomers have used lunar occultations to determine Mebsuta’s apparent diameter. When the moon slowly slides in front of a star the star does not just immediately blink out, as one might first think. Observed carefully (with a telescope, a precision light meter) the star fades as it is blocked by the moon then brightens as it reappears. The fading and brightening is more gradual the larger the star because the moon’s fine edge for a time doesn’t block all of the star’s surface.
By carefully observing lunar occultations of Mebsuta astronomers have deduced that its angular diameter is about four-thousandths of an arc second. As on a clock, sixty arc seconds make an arc minute and sixty arc minutes make a degree.
To be sure, Mebsuta appears very small. But when astronomers work out how big the star actually is by factoring in its distance from earth, the result is fantastic. Mebsuta is more than 130 times the size of our sun!
If Mebsuta were our sun, it would extend out to about the orbit of Venus. And, of course, our earth would be inhospitable.
But this isn’t quite the end of Mebsuta’s story.
On 8 Apr., 1976, Mars occulted Mebsuta. By again carefully measuring the way in which Mebsuta dimmed and brightened as Mars slid in front of it, astronomers, armed with Mebsuta’s diameter, were able to determine some details of the thin Martian atmosphere. Specifically, they found that the temperature of Mars upper atmosphere was a frigid -130 C. This measurement was directly confirmed in that same year when NASA’s Viking 1 and 2 space probes landed on the Red Planet.
Mebsuta’s story offers a glimpse at how scientists claw their way to a better understanding of the Universe. Have a look for yourself in the next few days.
Look for a new post on our about 6 April 2008.
