Theory Cepheids

At the effect of measuring the distance of stars to the Earth, 'parallax,' a trigonometric technique, turned the most reliable method for making astronomical distance measurements, and a practice long employed by land surveyors here on Earth. The diameter of Earth's orbit is the base of a triangle, each side of which is attained by Earth each 6 months, and the star is the apex where the triangle's sides meet. The lengths of the sides are calculated by accurately measuring the three angles of the resulting triangle. Astronomical parallax works reliably well for stars within a few hundred light-years of Earth. Even that way, for example, measurements of the distance to Alpha Centauri, the star system closest to our Sun, vary only by one arc second however

Cepheids are standard candles used by astronomers to measure distances in the Universe. Cepheids are variable stars with a period of variation 1-70 days and a luminosity range 0.1 to 2 magnitudes. Cepheids are usually middle-aged (between 10 and 300 million years old), intermediate-mass stars which come to pulse with a regular beat that is related to how bright they are (Cepheids come in two main classes, one much younger than the other). Such a unique trait allows thus astronomers to consider that pulse and figure out how bright the Cepheid intrinsically is -which is how bright it would look like should you stand right next to. Comparing the value with how bright the star appear in our sky, astronomers can determine how far away a Cepheid is. Cepheids were discovered like standard candles in 1908 by Henrietta Leavitt. That use of Cepheids was first famously performed by astronomer Edwin Hubble in 1924, leading to the revelation that our Milky Way Galaxy is just one of many in a vast cosmic sea. Cepheids also helped in the discovery that our Universe is expanding and galaxies are drifting apart. Astronomers lately found by 2011 that winds from a Cepheid star could blow off significant amounts of gas and dust, forming a dusty cocoon around the star that would affect how bright it appears. This, in turn, would affect calculations of its distance. The fact was found about delta Cephei, which is racing along through space at high speeds, pushing interstellar gas and dust into a bow shock up ahead as a nearby companion star happens to be lighting the area, making the bow shock easier to see. Wind of delta Cephei, on the other hand, is up to one million times stronger than the solar wind. Such processes may lead to that Cepheids shrink in mass as they are slowly consumed by their wind, making them not quite as standard as once thought. Up to 25 percent Cepheids observed have also been seen losing mass. The findings however will help to make even more precise measurements of the size, age and expansion rate of our Universe. RR Lyrae variables have the same property than Cepheids thus may also be used like candlesticks. Decades ago, a small galaxy member of the Local Group, IC 1613, helped astronomers work out how to utilise variable stars to chart the Universe

In terms of cosmological studies, Cepheids are too dim to be found in very distant galaxies. To calculate longer distances, astronomers came to choose a special class of exploding stars called Type Ia supernovae as all flare with similar luminosity and are brilliant enough to be seen far across the Universe. By comparing the apparent brightness of Type la supernovae and pulsating Cepheid stars, they could measure accurately their intrinsic brightness and therefore use those to calculate distances. Adam Riess, a astronomer leading one of both teams which discoverd dark energy, was the discoverer of those new candlesticks. Such supernovae further may be magnified through intervening galaxy cluster. Programs are en cours to determine whether the exploding stars remain dependable cosmic yardsticks across vast distances of space. There is an ongoing controversy about Type Ia supernovas however about whether they are caused by a white dwarf pulling so much material from a companion star that it becomes unstable and explodes, or by the merger of two white dwarfs. By 2002 the Hubble Space Telescope found that distant white dwarfs are also of use to estimate the age of the Universe. A particular class of Type Ia supernovae that occur near youthful stars can improve measurements with a precision of more than two times. Such explosions are likely the result of youthful white dwarfs

Polaris, the North Star is a Cepheid, with very small pulses. Polaris however features long-term variations of luminosity as it brightened over the last two centuries. It had further dimmed for the last few decades but it is now brightening again