Some celestial yardsticks are visible in the night sky

The stars Vega and 61 Cygni, as well as Delta Cephei, are near the zenith or point directly overhead. They can be seen from mid-northern latitudes at 8 p.m. SkySafari app image credit
I get asked a lot of questions, including "How can astronomers determine how far distant a star or distant galaxy is from Earth?" Three stars have been shown how to measure stellar distances in our sky this week: Vega (61 Cygni), and Delta Cephei (Delta Cephei).

At 8 p.m., as the sky gets darker, go outside and look up at the sky. Vega will be visible in the constellation Lyra, The Lyre. It shines with a bright, bluish tinge. It is the second brightest star in the evening sky.

Here's a quick reminder that, contrary to the 1997 movie Contact, the correct pronunciation for this star is Veega. This was not Vayga which was a subcompact car that was sold by General Motors in the 1970s.

Related: How to See the Brightest Planets in the Night Sky: When and Where?

Two views taken by NASA's Spitzer Space Telescope of the bright star Vega. Scientists discovered signs that a Neptune-sized exoplanet orbiting this star is scorching hot. (Image credit: NASA/JPL-Caltech/University of Arizona)

While Vega's presence is obvious, it cannot be denied that 61 Cygni, our second star, is just as striking. It shines at magnitude +5.2 in Cygnus, the swan constellation. This star is more than 100 times brighter than Vega so be sure to locate it.

If you aren't affected by light pollution, you can see it using a star chart. It's also a beautiful double star when viewed through a small telescope.

Delta Cephei is, however, in the constellation Cepheus, the king. This star pattern better resembles an old church with steeples. Delta is located in the southeast corner (lower left) of the church.

This view of the sky surrounding the bright star system Alpha Centauri is a wide-field one. It was created using photographic images from the Digitized Sky Survey 2. Image credit: ESO/Digitized sky Survey 2 Acknowledgement to Davide De Martin

Measuring distance using displacement

Vega, 61 Cygni and the southern star Alpha Centauri were the first stars whose distances could be measured using the trigonometric paraallax method. This was in the late 1830s. To demonstrate parallax, hold your thumb at arm's reach and cover an object such as a streetlight. Close your left eye, and use your right eye to cover the light with your thumb.

Close your right eye and then open your left eye. It will appear that you thumb has moved to the right side of the light. We can also measure the parallax for stars located relatively close to Earth. This is done by measuring various points along Earth's orbit and then using trigonometric function to determine their distances. Each star's orbital revolution every year causes it to trace a small ellipse of its parallactic orbit. Its angular size is the same as Earth's orbit.

Friedrich Bessel, in eastern Prussia, and Wilhelm von Struve, in Russia, measured the parallaxes for Vega (61 Cygni) using meticulous visual observations. Struve used a filar microscope, a special eyepiece that is used in astronomical telescopes to measure astrometry on a 9.6 inch (24 cms) refracting telescope. Bessel used a Heliometer, a refracting telescope equipped with a split lens. This was used to determine the distance between two stars.

Alpha Centauri is only visible from Hawaii and along Florida's Gulf Coast. Its parallax was determined using a mural circle in South Africa by Thomas Henderson. This type of meridian circle, although it is an old-fashioned one, was used to determine its parallax.

These were the first measurements of stellar distances. They were very close to the current measurements of 25.0 light years for Vega, 11.4 light years for 61 Cygni, and 4.37 light years for Vega. These were significant breakthroughs, and such observations were not made visually until around 1900. Then, improved photographic techniques were developed, which eventually led to automated plate-measuring machines linked to electronic computers.

In recent years, astronomers used the European Space Agency (ESA), scientific satellite Hipparcos from 1989 to 1993. "Hipparcos", an acronym for High Precision PARallax COllecting satellite, is the name of the word. It was the first satellite dedicated to precision astrometry. The satellite provided precise measurements of the positions and parallaxes stars, which allowed for better calculations of their distance and potential tangential velocity.

A shot focusing upon the Cepheid variable Star (Image credit to NASA, ESA and the Hubble Heritage Team, STScI/AURA).

Delta Cephei: Stellar Rosetta stone

The stars' parallaxes shrink to a point beyond 100 light years. Then, how is it possible to determine the distances between galaxies?

The "Rosetta Stone" of astronomy, Delta Cephei provided a clue to how to solve the problem. John Goodricke, an amateur English astronomer, discovered that the star's brightness would change over five days. Later, it was discovered that Cepheids were also able to do this. Today, there are over 400 Cepheids in the Milky Way. There are many more in nearby galaxies. Each star pulsates for a period of one to about 100 days. The light changes are repeated like a clockwork.

Cepheids possess a useful property. The Cepheids know that the longer the period, each star's intrinsic (actually) luminosity will be. This allows us to construct a period-luminosity graph. If a Cepheid variable in a distant star is detected, we can calculate its period and determine its absolute magnitude using the curve. We can then compare the star's apparent magnitude and absolute magnitude to accurately determine its distance. This is similar to how a lighthouse's distance can be calculated from its candlepower and how bright it appears.

There are two types of Cepheids

But things got confusing later. It was discovered that there were actually two types of Cepheids after the mid-1940s. Type I Cepheids are approximately one-and-a half times brighter than Type II, if taken period by period. Until then, the distances between galaxies was underestimated. My personal astronomical library contains a variety of books that date back to before 1950. Many of these books listed the Andromeda Galaxy as only 750,000 light years away. Later, this was revised to 1.5million light-years. However, we now know that it is at a distance just above 2.5 million light years.

The Andromeda Galaxy is located 2.5 million light years from Earth. It is the nearest major galaxy to the Milky Way. (Image credit: Shutterstock)

It's possible to see it for yourself

This is the prototype star, which made all of these advancements possible. It's an object you can monitor nightly. The Church-like star pattern of Cepheus shows Delta Cephei, which marks the acute angle of a triangle of stars. The other corners are Zeta Cephei and Epsilon Cephei (magnitude 3.4)

At maximum light, Delta looks very similar to Zeta. At minimum it is half as bright, which makes it a good match with Epsilon.

If you observe Delta Cephei for five consecutive nights, over a period of about a week you will be able to see its five-day cycle, which is a rapid brightening and slow decline. You can observe the brightness rise and fall with binoculars. Average pulsation time is 5 days, 8 hours and 47 min.

Joe Rao is an instructor at the Hayden Planetarium in New York. He is a writer on astronomy for Natural History magazine and the Farmers' Almanac, among other publications. Follow us on Facebook and Twitter @Spacedotcom