Super-sharp observations with the telescope Alma have revealed what seems to be a gigantic flare on the surface of Mira, one of the closest and most famous red giant stars in the sky, with its 13-light-year long tail. Activity like this in red giants - similar to what we see in the Sun - comes as a surprise to astronomers. The discovery could help explain how winds from giant stars make their contribution to our galaxy's ecosystem.
New observations with Alma have given astronomers their sharpest ever view of the famous double star Mira. The images clearly show the two stars in the system, Mira A and Mira B, but that's not all. For the first time ever at millimetre wavelengths, they reveal details on the surface of Mira A.
"Alma's vision is so sharp that we can begin to see details on the surface of the star. Part of the stellar surface is not just extremely bright, it also varies in brightness. This must be a giant flare, and we think it's related to a flare which X-ray telescopes observed some years ago", says Wouter Vlemmings, astronomer at Chalmers University of Technology, who led the team.
Mira - the name means "Wonderful" in Latin - has been known for centuries as one of the most famous variable stars in the sky. At its brightest, it can be clearly seen with the naked eye, but when it's at its faintest a telescope is needed. The star, 420 light years away in the constellation Cetus, is in fact a binary system, made up of two stars of about the same mass as the sun: one is a dense, hot white dwarf and the other a fat, cool, red giant, orbiting each other at a distance about the same as Pluto's average distance from the Sun.
"Mira is a key system for understanding how stars like our sun reach the end of their lives, and what difference it makes for an elderly star to have a close companion", says Sofia Ramstedt, astronomer at Uppsala University and co-author on the paper.
The Sun, our closest star, shows activity powered by magnetic fields, and this activity, sometimes in the form of solar storms, drives the particles that make up the solar wind which in its turn can create auroras on Earth.
"Seeing a flare on Mira A suggests that magnetic fields also have a role to play for red giants' winds", says Wouter Vlemmings.
The new images give astronomers their sharpest ever view of Mira B, which is so close to its companion that material flows from one star to the other.
"This is our clearest view yet of gas from Mira A that is falling towards Mira B" says Eamon O'Gorman, astronomer at Chalmers and member of the team.
The observations were carried out as part of Alma's first long-baseline observations. By placing the telescope's antennas at their maximum distance from each other, Alma reached its maximum resolution for the first time. Mira was one of several targets in the campaign, alongside a young solar system, a gravitationally lensed galaxy and an asteroid. Now Wouter Vlemmings and his team plan new observations of Mira and other similar stars.
Galaxy Evolution Explorer--"GALEX" for short--scanned the popular star during its ongoing survey of the entire sky in ultraviolet light. Astronomers then noticed what looked like a comet with a gargantuan tail. In fact, material blowing off Mira is forming a wake 13 light-years long, or about 20,000 times the average distance of Pluto from the sun. Nothing like this has ever been seen before around a star.
"I was shocked when I first saw this completely unexpected, humongous tail trailing behind a well-known star," says Christopher Martin of the California Institute of Technology. "It was amazing how Mira's tail echoed on vast, interstellar scales the familiar phenomena of a jet's contrail or a speedboat's turbulent wake." Martin is the principal investigator for the Galaxy Evolution Explorer, and lead author of a Nature paper appearing today to announce the discovery.
Astronomers say Mira's tail offers a unique opportunity to study how stars like our sun die and ultimately seed new solar systems. Mira is an older star called a red giant that is losing massive amounts of surface material. As Mira hurtles along, its tail sheds carbon, oxygen and other important elements needed for new stars, planets and possibly even life to form. This tail material, visible now for the first time, has been released over the past 30,000 years.
"This is an utterly new phenomenon to us, and we are still in the process of understanding the physics involved," says co-author Mark Seibert of the Observatories of the Carnegie Institution of Washington in Pasadena. "We hope to be able to read Mira's tail like a ticker tape to learn about the star's life."
Billions of years ago, Mira was similar to our sun. Over time, it began to swell into what's called a variable red giant - a pulsating, puffed-up star that periodically grows bright enough to see with the naked eye. Mira will eventually eject all of its remaining gas into space, forming a colorful shell called a planetary nebula. The nebula will fade with time, leaving only the burnt-out core of the original star, which will then be called a white dwarf.
Right: Click on the image to play an animated artist's concept of red giant Mira evolving its comet-like tail. [More]
Compared to other red giants, Mira is traveling unusually fast, possibly due to gravitational boosts from other passing stars over time. It now plows along at 130 kilometers per second, or 291,000 miles per hour. Racing along with Mira is a small, distant companion thought to be a white dwarf. The pair, also known as Mira A (the red giant) and Mira B (the white dwarf), orbit slowly around each other as they travel together through the constellation Cetus 350 light-years from Earth.
In addition to Mira's tail, GALEX also discovered a bow shock, a type of buildup of hot gas, in front of the star, and two sinuous streams of material coming out of the star's front and back. Astronomers think hot gas in the bow shock is heating up the gas blowing off the star, causing it to fluoresce with ultraviolet light. This glowing material then swirls around behind the star, creating a turbulent, tail-like wake. The process is similar to a speeding boat leaving a choppy wake, or a steam train producing a trail of smoke.
The fact that Mira's tail only glows with ultraviolet light might explain why other telescopes have missed it. GALEX is very sensitive to ultraviolet light and also has an extremely wide field of view, allowing it to scan the sky for unusual ultraviolet activity.
The Daily Galaxy via Chalmers University of Technology and NASA
Image credit: Katja Lindblom, CC BY-NC-ND 4.0 and NASA/GALEX