Thursday, November 25, 2010

Honk If You Love the Heliosphere

I’ve heard that there’s a point, marked by turbulence, that marks the edge of the solar system as a satellite passes out of it. What causes the turbulence? What is out there to be turbulent? ~Matt

Racing from the Sun in all directions at about a million miles per hour, the solar wind is a stream of electrically charged particles. The solar wind, carrying with it the Sun’s magnetic field, forms an enormous magnetic “bubble” called the heliosphere that encases the solar system. This protective bubble shields our solar system and planet from (some but not all) harmful cosmic rays, high-energy particles traveling through space. We heart the heliosphere!

The transition zone where the solar wind meets the interstellar medium, the thin gas and dust that exists between stars, is called the heliopause. Here at the outer edges of the heliosphere, the solar wind mixes with the interstellar medium; this interaction between two different densities and pressures creates turbulence.

The Heliosphere and the Voyager Spacecrafts
Courtesy NASA/JPL-Caltech

When the solar wind approaches the heliopause, it slows very abruptly, causing a shock wave to form. The shock wave is called the termination shock, and this boundary is marked by dramatic changes in magnetism and temperature.

NASA’s Voyager 2 spacecraft crossed the termination shock in 2007; Voyager 1 had crossed it three years earlier in 2004. And here’s a funny twist: Voyager 1 only had to cross the termination shock once; Voyager 2 had to cross it five times! This occurred because the heliosphere bubble flexes in response to solar flares and other ejections of material from the Sun. In this case, the termination shock became a moving finish line.

Voyager 1, launched in 1977 and now ten-billion-plus miles from Earth, is the most distant of all active spacecraft. Voyager 2, which launched a few weeks earlier than Voyager 1, is only eight-billion-plus miles from Earth. This lag exists because Voyager 2 took a little planetary side tour. It began heading out of the solar system nine years later than Voyager 1, after becoming the first spacecraft to observe Uranus and Neptune.

In about five years, Voyager 1 will leave the heliosphere behind and enter interstellar space; Voyager 2 a few years after that, each spacecraft heading in a different direction. What new wonders will these stalwart space voyagers encounter, in the realm beyond the Sun? Stay tuned.

Thursday, November 11, 2010

How Far Can We See? (part two)

Now for the exciting conclusion--and the second answer--to Matt's question of last week, which you may recall was:

"Is there a specific star that represents the farthest a person can see with the naked eye?"

Although the Andromeda Galaxy is the farthest object the average person can see with the naked eye, there is a star that is reportedly the most distant that can be seen naked-eye. It's a variable star in the constellation Cassiopeia the Queen, and it has the unromantic name of V762 Cas.

A variable star is a star that periodically brightens and dims. In the case of V762 Cas, it's most likely doing that because it's in a red supergiant phase, and it's begun to swell and shrink. A red supergiant is approaching the end of its life. It has burned up all its hydrogen fuel, and has begun burning other elements in a specific sequence. This causes rather dramatic changes in the star's size, temperature, and "behavior."

Image from Palomar Observatory Sky Survey

V762 Cas lies about 15,000 light years away. As we learned last week, one light year is nearly six trillion miles. Now multiply that by 15,000, take two aspirin, and go lie down.

V762 Cas must be a big honkin' supergiant, if it's that far away and we can still see it, yes? Unfortunately, I have not yet been able to spot V762. It is right on the outer limits of naked-eye accessibility, so I need to try again at a really dark site. Acquiring this sort of faint target, along with a faint target like the Andromeda Galaxy, benefit greatly from an effort to properly dark adapt (see last week's post).

To give it a try yourself, face north and locate the Lazy W asterism of Cassiopeia, which looks like a W or an M, depending on its orientation when you look. Download a free sky map for the month at, if you don't know how to locate Cassiopeia.

Now draw an imaginary line between the star at the roof peak of the House asterism of Cepheus--it looks like a child's drawing of a house--and the leftmost star of the Lazy W, if it were oriented as a right-side-up W. V762 lies on that line. The star map below will help you pinpoint its location.

Looking north toward Cassiopeia and Cepheus

If you have success spotting it, please post your results in the Comments. Also, if you can spot it with binoculars or a telescope, let us know that also; I'm curious to know if anyone sees any color in the magnified image. Sometimes color is significantly enhanced with a little magnification.

Good luck to us all!

Thursday, November 4, 2010

How Far Can We See? (part one)

Astronomy enthusiast Matt, whose favorite celestial object is the three-star asterism (recognizable star pattern) Orion’s Belt, dropped quite a few questions into the Ask An Astronomer box. We’ll start with this one, since it’s perfectly timed for autumn stargazing:

“Is there a specific star that represents the farthest a person can see with the naked eye?”

I love this question, because it has two answers. First, the most distant object that a person with average eyesight can see with the naked eye is…
...not a star! It’s a galaxy, specifically, the Andromeda Galaxy.

Originally called the Great Andromeda Nebula, it became known as the Andromeda Galaxy after the famous astronomer Edwin Hubble confirmed its galactic nature in the 1920s. At an estimated 2.5 million light years away, it’s the nearest spiral galaxy to our Milky Way. The Andromeda Galaxy is more or less comparable to our home galaxy in size and mass, but astronomers believe it has a significantly higher star count.

Keep in mind that one light year is nearly six trillion miles, so the distance to the Andromeda Galaxy is pretty hard for us to wrap our heads around. But amazingly, with a little bit of effort, we can see it! Let’s gaze.

1) You’ll need a dark site, away from city lights, to see this “faint fuzzy.” Before beginning your hunt, be sure to dark adapt to maximize the acuity of your night vision. To dark adapt, simply avoid all white light for 20 minutes before stargazing. Then use a red flashlight during your stargazing session, to maintain your dark adaptation.

2) About one hour after sunset, face east. Look well above the eastern horizon for a large, four-star square. Each side of the square is about two fists wide, if you hold your fist at arm’s length against the sky and measure across the knuckles. This is the asterism called the Great Square of Pegasus.

Star maps created with Your Sky

3) Next, find the Chains of Andromeda, the two strands of stars that arc to the left (north) of the star that marks the Square’s lower left corner. Now look above the middle star of the upper chain for a faint, fuzzy patch. A Persian astronomer of the 10th century called it the “little cloud,” an apt description. If you spot it, you’re gazing upon a collection of about one trillion gravitationally-bound stars, what the farsighted philosopher Immanuel Kant called an island universe.

If you can’t see it, you may need to try again at a darker site with less light pollution. Additionally, your sky transparency, or atmospheric clarity, may be negatively impacted by the presence of clouds, haze, dust, or humidity. If this is the case, try again when conditions are improved.

When you look up at the night sky, every star you see is in the Milky Way. The exciting thing about spotting the Andromeda Galaxy—in addition to it being the farthest object you can see naked eye—is that you’re seeing a world beyond your galactic neighborhood. That’s right, it ain’t local.

For the second answer to Matt’s fine question, tune in next week. And until then, happy hunting.