Expert: Ed - 6/19/2005

Question
As a student I have two question and I am asking you.
These two questions came to my mind:


1.Why we would not expect to find life in any planets circling around O5 class star?
2. Why do 5 solar mass stars form planetary nebulas,thus shedding their mass and allowing them to run into white dwarfs but 10 solar mass stars do not?

Thank you so much
Regards


Answer
Sammy,

Make sure you change the wording of this answer, if these questions are from a homework assignment.  Parts of them are taken from websites that your teacher could find, very easily.

O5 class stars are incredibly hot and bright.  They are blue-white in color, and they burn up so quickly that life doesn't even have time to form...  The O type star is the first and brightest in the sequence of spectral types, and are distinguished by lines of ionized helium.  Often neutral helium and weak hydrogen lines are also visible. An O5 type star generally has a mass of about 40 times that of Sol, a luminosity of 405,000 times as much, and and a surface temperature of some 40,000° Kelvin. It is so hot it burns blue-white in colour. Being so hot and bright this type of star burns up all its fuel very quickly. When it collapses, having exausted all its feul, an O type star will usually become a large black hole. An O5 will spend only about million years on the main sequence. These stars therefore are very rare, on average about one in every billion cubic light years of space. Only one star in 4 million is an O-type. Despite having an expansive life-zone, O-type stars never have life-supporting planets, as they do not endure long enough for life to form.

Planetary nebulae can only form in certain conditions.  

A planetary nebula is born when a low mass star dies (low mass means less than about 8 times the mass of the Sun), while the supernova is the death of a massive star.

In low mass stars, the outer envelope of the star is ejected out while the core of the star becomes a "white dwarf". The ejected envelope expands away from the central star and creates the nebula that we see. So, if you look at the pictures of classic planetary nebulae like the Ring nebula, you will be a ring of material with a star in the center. The ring of material was once part of the star in the center, but now has been ejected. The central star is now a white dwarf.

In high mass stars, the story is entirely different. Nuclear fusion in the core produces elements all the way to iron. Once the mass of the iron core exceeds a certain threshold, it collapses causing shock waves to propagate outwards. The end result is the titanic explosion called the supernova, leaving behind a neutron star or a black hole. The exploded remains of the star form a "supernova remnant". Examples are the Crab nebula (M1 in Taurus) and the Veil nebula in Cygnus. Hence, a supernova produces a supernova remnant and not a planetary nebula.  (Stars with 10 solar masses are DEFINITELY in this category!)

Here is a nice NASA link about supernovas (also called supernovae).
http://imagine.gsfc.nasa.gov/docs/science/know_l2/supernovae.html

I hope this is helpful.  Keep Looking Up!
--Ed