- 3191 - WHITE DWARF STARS - what happens when they merge? Astronomers have found a white dwarf star that was once two white dwarfs. The pair of stars merged into one about 1.3 billion years ago. The resulting star is about 150 light years away.
------ 3191 - WHITE DWARF STARS - what happens when they merge?
- A white dwarf is the end state of stars like our Sun. Once they have consumed their nuclear fuel, they expel their outer material. What is left is dense collection of matter, with no fusion taking place. It’s luminosity only comes from its stored thermal energy.
-
- This white dwarf is different than others. Normally, a white dwarf’s atmosphere doesn’t contain much carbon. This white dwarf has an atmosphere with elevated levels of carbon, and it is much more massive than a typical white dwarf star.
-
- Before a star becomes a white dwarf, it goes through a “red giant” phase. If the red giant doesn’t have enough mass to fuse carbon, then carbon and oxygen will collect in a mass at the star’s center.
-
- As the star sheds it outer layers, it eventually leaves behind only a remnant, which is the white dwarf. Most dwarfs are primarily composed of that carbon and oxygen.
-
- Normally that carbon is not visible. A thick layer of helium usually blocks it. Spectroscopic measurements in this case showed that the atmosphere had an unusually high carbon content, something that should not be possible.
-
- This star stood out as something we had never seen before. You might expect to see an outer layer of hydrogen, sometimes mixed with helium, or just a mix of helium and carbon. Astronomers don’t expect to see this combination of hydrogen and carbon at the same time as there should be a thick layer of helium in between that prohibits that.
-
- Most white dwarfs are not that massive. They are typically about 0.6 times as massive as our Sun. This white dwarf is different. Its mass is almost double that at 1.14 solar masses. It’s still extremely compact for an object with that much mass. It’s only two-thirds the diameter of Earth.
-
- It’s velocity through space also doesn’t match up with other white dwarfs. Since older stars travel faster than young ones as they both orbit in the Milky Way, this white dwarf must older. In fact, its high velocity means its faster than 99% of other white dwarfs in the galaxy.
-
- Because stellar velocity dispersion increases with system age, the fast kinematics of this white dwarf may signify a system age much older than implied simply from the white dwarf cooling.
-
- All three characteristics set it apart from other white dwarfs: its velocity/age, its mass, and its visible carbon. The only way you can explain it is if it was formed through a merger of two white dwarfs.
-
- The merged-dwarf is likely the outcome of billions of years of stellar evolution in a binary star system. One of the star’s reaches its red giant phase before its partner, and expands, enveloping the partner. As the first star then shrinks, the orbit between the two draws closer. Then the second star goes through its red giant phase, expanding and enveloping the other.
-
- It takes billions of years, but eventually the “gravitational wave emission” makes the orbit shrink even more. When the orbit shrinks sufficiently, the stars merged into one.
-
- There is an upper mass limit to white dwarfs, even for a pair that have merged. If the resulting stellar object is sufficiently massive, it will explode as a thermal runaway supernova.
-
- This limit is about 1.4 solar masses. It’s possible that objects can explode as supernovae at less than 1.4 solar masses. At 1.14 solar masses, this white dwarf is helping astrophysicists understand the upper mass limit.
-
- Scientists can understand a white dwarf’s age by looking at its temperature. White dwarfs don’t generate any heat because there’s no fusion anymore. They’re more like embers than stars, and by monitoring the star’s cooling, they can determine its age. But when two white dwarfs merge, the cooling process starts over.
-
- There’s no precise way to determine this one’s age, and the two white dwarfs may have been white dwarfs for billions of year before merging. Still, the researchers think that the merger itself took place about 1.3 billion years ago.
-
- Scientists have only ever found a few merged white dwarfs before. But this is the first time one has been identified through its composition. If two white dwarfs are massive enough when they merge, they'll explode as a supernova.
-
- Maybe the most exciting aspect of this star is that it must have just about failed to explode as a supernova. These gargantuan explosions are really important in mapping the structure of the Universe, as they can be detected out to very large distances.
-
- There remains much uncertainty about what kind of stellar systems make it to the supernova stage. Strange as it may sound, measuring the properties of this ’failed’ supernova, and future look-alikes, is telling us a lot about the pathways to these thermonuclear explosions.
-
- June 13, 2021 WHITE DWARF STARS - when they merge? 3185
----------------------------------------------------------------------------------------
----- Comments appreciated and Pass it on to whomever is interested. ----
--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
-- email feedback, corrections, request for copies or Index of all reviews
--- to: ------ jamesdetrick@comcast.net ------ “Jim Detrick” -----------
--------------------- --- Sunday, June 13, 2021 ---------------------------
No comments:
Post a Comment