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According to Wikipedia, a rogue star or intergalactic star is

not gravitationally bound to a galaxy.

But according to a citation on the same Wikipedia page, the first intergalactic stars were discovered in the Virgo cluster of galaxies.

These stars form a massive group aproximately 300,000 light years away from the nearest galaxy.
(Emphasis mine)

Is there any clear, stated definition which differentiates galaxies from "massive groups of rogue stars"?

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    $\begingroup$ I tried to clean up your question to have better links and quotes. I believe I maintained the spirit and meaning of your question but if not, feel free to roll back. $\endgroup$ – zephyr Jul 14 '17 at 14:22
  • $\begingroup$ No problem. Anyway, I think the main definition is missleading too. Any star is at least very weakly gravitationally bound to the closest galaxy, isnt it? or how does that work? $\endgroup$ – Pablo Jul 14 '17 at 14:27
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    $\begingroup$ Not necessarily. If it's velocity wrt to the galaxy is greater than the escape velocity of that galaxy, then we say it is not gravitationally bound. For example, the escape velocity of the Milky Way is several hundred $km/s$ (how much it is exactly depends on a variety of factors). Basically, if the star's motion away from the galaxy can never be slowed to a stop by that galaxy's gravitational pull, we consider it gravitationally unbound. Certain hypervelocity stars are certainly traveling too fast to be bound and can become "rogue" once they leave their host galaxy. $\endgroup$ – zephyr Jul 14 '17 at 14:32
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It is just a poor choice of words. The comment refers to a discovery paper by Ferguson et al. (1998), where they used a very deep image in the Virgo cluster to establish that there were an excess of distant stars compared with a control field outside of the cluster.

This excess of about 630 stars were all in one tiny HST field of view, so form a "group" in some sense, but there is no suggestion that these stars are not found throughout the Virgo cluster. Indeed subsequent work has found evidence for intracluster stars in other areas of the cluster (e.g. Durrell et al. 2002).

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According to theoretical work by Dr. Ernest Sternglass, there are expected to be many such groups of "rogue stars". In Sternglass's model [the "electron-positron pair model of matter"] there are expected to be many stars and clusters of stars which are not near any galaxy because of the process by which stars and star clusters form.

Briefly, the model says that each kind of astronomical object, such as a single star, or a star cluster, or a small galaxy, or a large galaxy, or a galaxy cluster, etc., is formed by a single massive explosion, (known as a "quasar"). A not-very-energetic quasar might form a star cluster, while a very-energetic quasar might form a large galaxy.

In other words, the model says that a quasar is a galaxy or star cluster, or other astronomical object, in the process of being born.

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    $\begingroup$ Your answer would probably be improved with a link to Dr. Sternglass' work, but off hand, the idea of a quasar forming a galaxy sounds very far from the truth. $\endgroup$ – userLTK Jul 18 '17 at 23:34
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    $\begingroup$ Sternglass published a number of papers during his career. His "main" paper, as he described it to me, is titled: "Relativistic Electron-pair Systems and the Structure of Neutral Mesons", Physical Review, July 1, 1961. I'll list four more of his papers in my next comment. Plus, he wrote a book, for scientists and also for the general public, to summarize his life's work, including his "electron-positron pair model of matter". The book is titled Before the Big Bang (1997, 2001) and is available at www.AMAZON.com. $\endgroup$ – PERFESSER CREEK-WATER Jul 20 '17 at 0:19
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    $\begingroup$ Here are three more of Sternglass's published papers: (1) "New evidence for a molecular structure of meson and baryon resonances", from the Proceedings of the 2nd Resonance Particles Conference, Athens, Ohio, 1965; (2) Electron-pair theory of meson structure and the interactions of nuclear particles", Proceedings of the American Physical Society (1964), osti.gov/scitech/biblio/4885112; (3) "Electron-positron model for the charged mesons and pion resonances", Il Nuovo Cimento 35(1): 227-260 (December 1964); $\endgroup$ – PERFESSER CREEK-WATER Jul 20 '17 at 0:29

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