How Long For a Star to Lite Up?

[litespeed]

Specifically, from first fusion to full burn. For instance, does hydrogen continue to accumulate after lite up, or does the solar wind blow it away. I assume it does not, since stars come in many sizes.

[maddog]

Specifically, from first fusion to full burn. For instance, does hydrogen continue to accumulate after lite up, or does the solar wind blow it away. I assume it does not, since stars come in many sizes.

There are a couple of depends here...

 

1. First it depends on how big a star is.

2. Second would the environment in which the star formed.

 

For example wrt #1, our star is a main sequence (V) G2 (roughly) type of star. So our star would last about 10 billion years through the hydrogen burn life. It is thought to be a bit over 5 billion years old at the moment which means we have about half left. Then our star will move into the Red Giant phase and balloon out past Earth's orbit and engulf our planet.

 

Were our planet further down the main sequence say a M5 dwarf like a neighbor Barnard's star (about 18 light years away). It's main hydrogen burn could be significantly longer. Likewise in the other direction a star like Sirius (A0, another neighbor) could go through it's main burn before two billion years were up. It is thought that some O stars may have a lifetime in their hydrogen burn cycle of only 50 million years or so.

 

The second depends is about where a star forms. Our star is on the edge of the arm of one of the spirals of our galaxy. As such our local stellar neighborhood is modestly dense. In some globular clusters this may be many more time compact such that a planet their might not ever have daylight. This can attenuate the lifetime of the hyrdogen burn one way or the other.

 

I hope this answered your question..

 

 

maddog

[litespeed]

Thanks for the reply maddog. Very informative. However, my primary interest is how long it takes a new star to go through the initial fussion stage and reach its full brightness. This is something different then how long it will burn AFTER full fusion is achieved.

 

Any thoughts?

 

Thanks again!

[modest]

How Long For a Star to Lite Up? Specifically, from first fusion to full burn.

 

The life cycle of a star is from interstellar (molecular) cloud -> protostar -> pre-main-sequence star (called a T Tauri star if it is about the mass of our sun) -> main sequence star.

 

From the time fusion starts until the star becomes main sequence is in the order of 10 million years (for a star of the mass of our sun). Some sources I'm looking at say "a few million years" and some say "in the order of tens of millions of years" referring to the entire collapse / protostar process. But, it looks like approximately 10 million is the best answer “from first fusion to full burn”

 

Step 3 quoted here says it concisely:

3. Once the core temperature reaches 10 million K, coulombic repulsion between the now ionised hydrogen atoms (protons) is overcome and nuclear fusion commences. Hydrogen fuses to form helium nuclei, releasing energy in the process. Initially the increased outward radiation pressure is still insufficient to halt gravitational collapse but it does slow it down. The star's surface temperature increases significantly, compensating for the drop in size so that its luminosity increases slightly. The star's track moves up slightly and to the left on the H-R diagram over

10 million years

.

 

Stages in the formation of a 1 solar mass star

Where step 4 is then main sequence.

 

This time frame is for a sun-like star with a mass about equal to our sun. If the mass of the collapsing nebula and resulting object is less massive than our sun then it will take longer. If it is very small then the "sub-protostar" will never begin fusion at all, but will result in a brown dwarf. A protostar of much greater mass than our sun will reach its main sequence phase in only a few hundred thousand years. You can see this here:

 

As the central temperature and density continue to rise, the Nuclear fusion cycles become active, and the development of the (now genuine) star is stabilized. The star then is said to have reached the main sequence where it remains for most of its active life. The time required for the contraction phase depends on the mass of the star. A star of the Sun's mass generally requires tens of millions of years to reach the main sequence, whereas one of much greater mass might take a few hundred thousand years

 

The Evolution of Protostars

So, the best answer seems to be about 10 million years for a star about like our sun and would be more or less depending on it's mass.

For instance, does hydrogen continue to accumulate after lite up, or does the solar wind blow it away. I assume it does not, since stars come in many sizes.

 

After fusion starts, and during the protostar and the pre-main-sequence phases, the core definitely continues to accrete matter. The last step before main sequence is a T Tauri star and you can see from that link they have both accretion discs and solar wind. So, there is solar wind blowing away some of the lighter material, but there is also accretion happening.

 

~modest

[litespeed]

Modest:

 

Thanks for the best summary I have seen on this matter; comprehensive and concise as well.

 

While we are on the subject, what do you understand to be the smallest star possible. In this deffinition, I include any body that produces self sustaining fusion lasting longer then, say, 10,000 years.

[maddog]

While we are on the subject, what do you understand to be the smallest star possible. In this deffinition, I include any body that produces self sustaining fusion lasting longer then, say, 10,000 years.

I seem to remember an estimate of about 30 Jupiter masses for the smallest star to ignite Hydrogen. This would be like an M8 main sequence star. Modest might have more recent data. Mine is about 15+ years out of date.

 

maddog

[belovelife]

my theory

 

Now according to this theory (completely unfounded :) ), the only limit to minimum star size would

be the size of the ejected matter from the black hole.

Dr. Eric Pfahl, University of Virginia 06

Dr. Eric Pfahl, University of Virginia 06

Screen clipping taken: 1/20/2009, 8:57 PM

 

Although the ejected matter may fizzle in millisec. depending on the size.

Larger objects would assume an orbit, collide with other objects, or completely have

a vector away from the center of the galaxy.

[modest]

The best info I can find is in the same ballpark as Maddog's,

 

Below 0.072 solar masses, the thermonuclear fusion of hydrogen becomes impossible. The objects immediately below this critical mass are called brown dwarfs.

 

The Astrophysics Spectator: Brown Dwarfs

 

A different site said .075 solar masses. These equate to about 80 times the mass of Jupiter. But, it's probably important to note that lighter brown dwarf's can fuse whatever deuterium and lithium there is available to it. From wikipedia's article on brown dwarfs, objects greater than ~13 Jupiter masses will fuse deuterium and greater than ~65 will fuse lithium.

 

So, it looks like less than 13 times the mass of Jupiter is a giant planet. Greater than ~13, but less than ~80 times Jupiter's mass is a sub-stellar object (not quite a real star, but a brown dwarf), and greater than 80 Jupiter masses (or 0.072 solar masses) is considered a proper star (a red dwarf star).

 

~modest

[litespeed]

This has been a good discussion. Lots of concise info. I will move on to Neutron Stars as a topic.