OK, make that 25 million light years. It’s still a long way to go and it is yet amazing that a light shining that far away is visible from my backyard deck. Karl J. Runge was thoughtful to point out that a reference in the last column to the galaxy M101, where the supernova is blowing its top, isn’t 250 million light years away, but only about 25 million. I strive to be accurate, and it is a wonder what a little misplaced “zero” can do. Isn’t zero supposed to represent nothing? Yet what a mistake it can make on your checkbook or in talking about how far away a galaxy is shining. We can always blame the decimal point for moving.

OK, make that 25 million light years. It’s still a long way to go and it is yet amazing that a light shining that far away is visible from my backyard deck.


Karl J. Runge was thoughtful to point out that a reference in the last column to the galaxy M101, where the supernova is blowing its top, isn’t 250 million light years away, but only about 25 million. I strive to be accurate, and it is a wonder what a little misplaced “zero” can do. Isn’t zero supposed to represent nothing? Yet what a mistake it can make on your checkbook or in talking about how far away a galaxy is shining. We can always blame the decimal point for moving.


The supernova, by the way, continues to impress as it has stayed at nearly +10th magnitude for several weeks. A star of that magnitude is easily seen on a dark night in even a 3 inch telescope. Once the Moon light was out of the way, this past week I was pleased to see the actual galaxy, appearing as a faint, round smudge, with the supernova next to it.  Normally you could not hope to see individual stars in a far away galaxy, even if it is merely 25 million light years distant.


How can we tell how far away a star is from the Earth, let alone a galaxy? The easy answer is you look it up in a book or read a column and hope it’s right. That’s like asking how a radio works and you demonstrate by flipping a switch.


For thousands of years, sky observers did not realize that the stars or the Milky Way Band varied in distance. They all appear to be fixed on the great inverted dome of the sky. It was clear that the wandering planets, the moon and the sun were closer, and by their motions and speed we could detect the order of distance, out to Saturn, the most distant planet known before the telescope was invented.


Astronomers don’t limit themselves to expressing cosmic distance in millions or billions of miles or kilometers. Units of measurement include the Astronomical Unit (AU), which is the average distance of the Earth to the sun (93 million miles). This is handy for discussing distance within our solar system.


The term “light year” is often misused, as it is not an expression of length of time. Instead, it is the distance that light travels in one Earth year, traveling at approximately 186,000 miles a second. A light year then is roughly 5.8 TRILLION miles. Space is really vast by our standards. Our planet is not even quite 8,000 miles wide. The closest star system to the sun is Alpha Centauri, and that is 4.3 light years.


Distances within a few hundred light years are accurately measured by the parallax method. Using basic trigonometry, the star’s position is examined once and then again six months later, when the Earth is on the other side of the sun. This makes a rather big baseline (2 AU). From these two points, the star is seen to make a small shift in reference to the much farther background stars. How much of an angle of shift it makes is the star’s parallax. This is then used to compute the distance, since we already know how far we are from the sun.


Talking about cosmic distances, astronomers find light years too cumbersome, and instead refer to parsecs. One parsec equates to 3.26 light years. This is the distance of a star that has a parallax of two “arc seconds.” The sky is 360 degrees around, like any circle. Each degree is subdivided into 60 “minutes of arc.” the moon, for example, appears about one half degree, or 30 minutes of arc. Each of those tiny minutes of arc is further subdivided into 60 “seconds of arc,” a very tiny measure requiring a telescope to resolve.


The farther the star, the smaller the parallax shift.


Greater distances beyond the resolving power of our instruments, are figured through the shift of the colored spectrum of the starlight, or light from a galaxy.


Observation of a class of variable star, the Cepheid variable, also tells us distance by measuring how bright the star appears. A Cepheid variable star changes by a predictable amount; distance to closer Cepheid stars are known by parallax. Distance to others much farther away can be shown then by seeing how faint the star appears compared to nearer examples. Cepheid variable stars have been detected in closer galaxies, giving us a way to extend our celestial tape measure beyond the Milky Way.


New moon is on Sept. 27.


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Keep looking up!