Passing Stars

[BearOfNH]

On American cable and satellite TV there are a number of astronomy / cosmology programs speaking to the creation of our solar system, and various means of our eventual destruction. One method of destruction involves disturbing the Oort cloud, freeing a comet to fall in to the inner solar system and >BLAM< smash into the earth. If Jupiter doesn't catch it first, etc.

OK, but how does the Oort cloud get so disturbed in the first place? This is where it gets annoying. One standard excuse on these TV shows is that a "passing star" disturbs the otherwise peaceful comet cloud. What exactly is a "passing star"? I'm going to assume it's a star that passes by the solar system within, say, 4.3 ly distance. I doubt anything farther out than AC will affect the Oort cloud. (Or if it does, we're probably all doomed and these definitions won't matter anyway).

I took astronomy in college (over 40 years ago) but never heard of "passing stars" in that course. I can see how a "passing star" might show up now and then as the solar system winds its way across a galactic arm. But intuitively that doesn't seem too frequent an event. Is there any other way to disturb the Oort cloud?

If they're real, how frequent are these passing stars? Astronomers discover things and announce and catalog them all the time. But I have never heard announcements of any passing stars being discovered. Has anybody actually found a passing star in our lifetimes? Is there a catalog of such stars? Is anybody specifically looking for them?

I guess this is what bothers me. Scientists of high repute (e.g., Michio Kaku, Alexei Filippenko) call upon things that may never happen to illustrate ... ... wait for it ... ... how something might happen.

[swansont]

The gravitational effects may be small, but they would be present over a long time, and with a many-body system, small perturbations may be all that's necessary.

 

Plus, it may be that "passing stars" are only a small part of the picture

http://www.americanscientist.org/issues/pub/perturbing-the-oort-cloud

[BearOfNH]

http://www.americanscientist.org/issues/pub/1998/4/perturbing-the-oort-cloud

 

Thanks for the pointer, 00007. The article is well-written and concise. The thrust of the article is that when the solar system crosses the plane of the galaxy (every 30-35 million years), gravitational effects increase enough to where there's a lot more Oort cloud perturbations possible, and this can lead to a storm of comets heading into the inner solar system. (An "Oort storm"?)

 

But I wasn't asking about the Oort cloud specifically. That's just an example I called upon to show where the term "passing stars" was used. Maybe I could have phrased it better, but I really wanted to know more about "passing stars". There seems to be little in the way of hard evidence these things exist. One can make statistical arguments, but there seems to be little in the way of observational evidence.

 

And therefore it seems somewhat disingenuous for astronomers to pull "passing star" out of a hat when, for all we know, there hasn't been even one since the formation of the solar system.

[Spyman]

But we can measure the redshift in the light from nearby stars and conclude that they are moving away or towards us with different speeds which suggests that our Solar system and other stars are passing by each others, as they orbit in the Milky Way. We also have the ability to view other similar galaxies and model how stars behave in their large structure.

It is somewhat unfair to say that we don't have evidence of any *passing stars* when we don't have been advanced long enough to be able to observe and record such an encounter.

Below is a picture from Wikipedia that shows how, astronomers predict based on current knowledge and observations, the nearest stars will vary in distance over the next 80 thousand years.

Future and past
Ross 248, currently at a distance of 10.3 light-years, has a radial velocity of −81 km/s. In about 31,000 years it may be the closest star to the Sun for several millennia, with a minimum distance of 0.927 parsecs (3.02 light-years) in 36,000 years. Gliese 445, currently at a distance of 17.6 light-years, has a radial velocity of −119 km/s. In about 40,000 years it will be the closest star for a period of several thousand years.

Distances of the nearest stars from 20,000 years ago until 80,000 years in the future
http://en.wikipedia.org/wiki/List_of_nearest_stars#Future_and_past

In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, but they do so with constant angular velocity. The spiral arms are thought to be areas of high-density matter, or "density waves". As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars.
http://en.wikipedia.org/wiki/Galaxy#Spirals

Another interesting aspect is the so-called "wind-up problem" of the spiral arms. If the inner parts of the arms rotate faster than the outer part, then the galaxy will wind up so much that the spiral structure will be thinned out. But this is not what is observed in spiral galaxies; instead, astronomers propose that the spiral pattern is a density wave emanating from the Galactic Center. This can be likened to a moving traffic jam on a highway—the cars are all moving, but there is always a region of slow-moving cars. This model also agrees with enhanced star formation in or near spiral arms; the compressional waves increase the density of molecular hydrogen and protostars form as a result.
http://en.wikipedia.org/wiki/Milky_Way#Spiral_arms

Edited March 5, 2013 by Spyman

[Airbrush]

How about wondering Jupiter-sized planets? There are estimated to be more wondering planets than stars in our galaxy, according to last night's episode of "How the Universe Works". These could disturb the Oort cloud also.

[SciFiReal]

How about wondering Jupiter-sized planets? There are estimated to be more wondering planets than stars in our galaxy, according to last night's episode of "How the Universe Works". These could disturb the Oort cloud also.

Their gravitational effect can't be compared to the one of a whole star, imo. The mass of all our planets are about 1 to 100 in favour our Sun so planets can have almost no effect.

[Spyman]

Rogue planets are very hard to detect which causes a large range of estimations upwards to a very high number of free-floating planets for every star in the Milky Way. But even if they are more or less everywhere, Jupiter sized planets only contain a fraction of the mass of a normal star, (from one thousandth up to one hundredth of the Sun), and thus needs to come much closer to disturb the Oort cloud.

 

 

Observation

When a planetary-sized object passes in front of a background star, its gravitational field causes a momentary increase in the visible brightness of the background star. This is known as microlensing. Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues, who form the Microlensing Observations in Astrophysics (MOA) and the Optical Gravitational Lensing Experiment (OGLE) collaborations, carried out a study of microlensing which they published in 2011. They observed 50 million stars in our galaxy using the 1.8 meter MOA-II telescope at New Zealand's Mount John Observatory and the 1.3 meter University of Warsaw telescope at Chile's Las Campanas Observatory. They found 474 incidents of microlensing, just 10 of which were brief enough to be planets of around Jupiter's size with no associated star in the immediate vicinity. The researchers estimated from their observations that there are nearly two free-floaters for every star in our galaxy. Other estimations suggest a much larger number, up to 100,000 times more free-floating planets than stars in our Milky Way.

http://en.wikipedia.org/wiki/Rogue_planet#Observation

 

Planetary-mass objects

A planetary-mass object, PMO, or planemo is a celestial object with a mass that falls within the range of the definition of a planet: massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain core fusion like a star. By definition, all planets are planetary-mass objects, but the purpose of the term is to describe objects which do not conform to typical expectations for a planet. These include dwarf planets, the larger moons, free-floating planets not orbiting a star, such as rogue planets ejected from their system, and objects that formed through cloud-collapse rather than accretion (sometimes called sub-brown dwarfs).

http://en.wikipedia.org/wiki/Planetary-mass_object#Planetary-mass_objects

 

Brown dwarfs are substellar objects too low in mass to sustain hydrogen-1 fusion reactions in their cores, unlike main-sequence stars, which can. They occupy the mass range between the heaviest gas giants and the lightest stars, with an upper limit around 75 to 80 Jupiter masses (M_J). Brown dwarfs heavier than about 13 M_J are thought to fuse deuterium and those above ~65 M_J, fuse lithium as well.

 

However, for some years now there has been debate concerning what criterion to use for defining the separation between a brown dwarf and a giant planet at very low brown dwarf masses (~13 Jupiter masses). One school of thought is based on formation, and another on interior physics.

http://en.wikipedia.org/wiki/Brown_dwarf

 

A sub-brown dwarf is an astronomical object formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud) but that has a mass below the limiting mass for thermonuclear fusion of deuterium (about 13 Jupiter masses) and that hence is not a brown dwarf.

http://en.wikipedia.org/wiki/Sub-brown_dwarf

 

Jupiter is the fifth planet from the Sun and the largest planet in the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in the Solar System combined.

http://en.wikipedia.org/wiki/Jupiter

Edited March 6, 2013 by Spyman