If you look to the west at nightfall tonight you’ll notice that Venus has a close companion, the star El Nath. El Nath sits at the tip of Taurus the Bull’s northern horn; its name comes from the Arabic “al nath” meaning “the butting one”. Fitting for a bull, I’d say.
Over the past week or so, Venus and the star have been closing in toward each other. Tonight the two will be closest at only 0.8 degrees apart.
El Nath is one of only two “linking stars” in the sky. Linking stars officially belong to one constellation but do double duty filling out the outline of another. El Nath, also called Beta Tauri, completes the familiar pentagonal outline of Auriga the Charioteer directly above Taurus. The other one, Alpheratz (AL-fer-ratz), is Andromeda’s brightest star and completes the well known diamond-shaped Great Square of Pegasus. Without Alpheratz, the Square would collapse into an ordinary triangle.
El Nath is also known as Beta Tauri – the second brightest star in Taurus – and shines a bright second magnitude. That hardly compares to Venus’ brilliance. If we could somehow be transported to an imaginary planet orbiting El Nath, we’d be taken aback. Blazing before our eyes would be a star 4.6 times the size of the sun and shining 700 times brighter.
Its true brilliance is masked by the 130 light years that separate us. Venus wins out by proximity at a trifling 38 million miles.
Determining whether a star is bright because it’s closer or because it’s intrinsically brighter has been one of astronomy’s key questions since the invention of the telescope. How do you tell the difference?
We had to wait until 1838 before astronomers had the precision telescopes needed to measure a star’s parallax or shift against the more distant background stars. From that shift, the nearby star’s distance could from Earth could be determined (top). Parallax allowed us to extend our reach to about a 100 light years. Stars beyond that were compared to similar-type stars with known distances to extend the distance scale even further.
In 1908, Henrietta Leavitt discovered that variable stars called Cepheids could be used as “yardsticks” to measure distances as far as neighboring galaxies. Thanks to these efforts and others, nowadays we know which stars are truly bright and which are truly faint.