Grin at Mercury!

Not only can you smile at Saturn tomorrow (Friday, July 19), but you can also grin at Mercury.  The scientists using the MESSENGER mission to study the closest planet to the Sun will also capture Earth photobombing the view of the region around Mercury. They’re actually looking for any natural satellites the planet may have and realized that Earth will also be making a cameo appearance in images taken on Friday and Saturday  at 7:49 a.m., 8:38 a.m. and 9:41 a.m. EDT (11:49, 12:38 and 13:41 UTC). If the Sun is in the sky during any of these times and reasonably well above the horizon, then it’s your time to shine and wave for the cameras. Just don’t look at the Sun!  Mercury is about 12 degrees from the Sun, too close to be seen with the unaided eye.

Don’t forget to wave for Saturn, too. Check out my previous entry for details!

If you haven’t heard of MESSENGER before, the name stands for MErcury Surface, Space Environment, GEochemistry, and Ranging mission, and it sent the the first spacecraft to orbit Mercury. It has been studying the planet closeup for a few years now, and in that time has mapped the surface and provided many new insights into Mercury’s cratering rates, internal structure, and “weather” characteristics. Because the surface is so heavily cratered, it’s clear that the planet hasn’t been active (geologically) for billions of years (since the period called the Late Heavy Bombardment. Mercury is a very dense planet, with dense iron-rich core that is very likely still molten. Nobody expected Mercury to have much of an atmosphere, but it turns out that it does have a very tenuous cloud of material around it made up of hydrogen, helium, sodium, calcium and potassium. It’s very likely that this so-called “exosphere” is continually replenished by interactions between the solar wind and the surface of the planet.  There’s also a possibility that water exists on Mercury, but only in the polar regions that never get any direct sunlight. Elsewhere on the planet, surface ice wouldn’t last long; surface temperatures range upwards to around 800° F during the day!  In the early morning times before sunset, the surface temperatures plunge to close to -300!

If you want to know more about MESSENGER‘s explorations of Mercury, check out the mission website. It’s chock full of images and news releases outlining the amazing things planetary scientists are finding out about the closest planet to the Sun.

Sun-like Stars Die Beautifully

I’ve talked about planetary nebulae many times on this blog. They fascinate me,  just as starbirth regions do. They both represent stars at opposite ends of their lives. Planetary nebulae are old stars in designer wardrobes made for their funerals. As a star like the Sun gets old and begins the long descent into old age and death, it loses much of its mass to space. That mass forms a cloud of gas and dust surrounding the remains of the star, which is slowing contracting to become a small, hot, massive object called a white dwarf.  The white dwarf heats up the material in its shroud, and that causes it to glow.

The design of a white dwarf’s burial shroud can be quite complex. Take the object called NGC 2392, or as it’s more commonly known, the Eskimo Nebula. As you can see in the image at the left, the  material surrounding the star was ejected long ago and forms what looks like the outer hood of an Eskimo’s hood. The central region is a set of concentric shells and criss-crossing clouds of gas that have led scientists to suspect there’s more than one star at the heart of this object. X-ray emissions coming from the center of the nebula and detected by the Chandra X-Ray Observatory also suggest there’s more than one star here, probably a pair of binary stars.

Our own Sun will die like this in about 5 billion years. It probably won’t look quite so intricate, but the process will be the same. First the Sun’s mass loss (through a speeded-up 50,000 kilometer-per-hour solar wind) will cause it to shed huge amounts of its outer atmosphere to space. It will swell up to become a red giant, possibly reaching out to about the orbit of Earth. Our planet, Venus, and Mercury probably won’t survive, but Mars or the moons of the outer solar system could have a short-lived renaissance where their atmospheres could swell and water might flow freely on their surfaces.

Eventually, the dying Sun will contract to become a dense white dwarf, which will light up the surrounding clouds. The rest of the solar system will cool and die as the Sun does. Tens of billions of years from now there will be no cloud left, just a slowly cooling white dwarf and its dead retinue of worlds. The cloud of gas and dust will have spread out to space, lending itself to the inventory of material available to create new generations of stars and planets.

Astronomers study planetary nebula such as the Eskimo, the Ring, and others, in different wavelengths of light. Each wavelength gives them a good idea of the processes occurring in the nebulae, the speeds of stellar winds, and the action at the cores of these ancient, dying stars. Their insights help us all understand what will eventually happen with the Sun.