Full transcript
0:00so we're giving a question and we're
0:04told to estimate the mass of a star that
0:06has an earth-like planet orbiting at a
0:08distance of 1.5 a you remember that's
0:11our semi-major axis and that's
0:13astronomical units and we are told it
0:16takes to earth years to do an orbit
0:18great so we have a semi-major axis we
0:22have the orbital period and what we want
0:24is the mass and we're gonna get the mass
0:26in terms of stellar units how many times
0:29the mass of our Sun it is and that's why
0:32we're using earth you properties here so
0:35main an thing to keep in mind for any
0:38problem like this we're assuming m1 plus
0:41m2 is basically the mass one so when
0:45really I saw a distant star the mass of
0:48the star versus the mass of the star in
0:50the Sun are basically the same thing is
0:52what we're saying because the star has
0:53so much more mass than the planet the
0:56mass of the planet becomes more or less
0:58than significant so we can take that
1:01previous format and rearrange it to get
1:02this and this is gonna be true every
1:05time we just arbitrarily chose what mass
1:081 to be the star
1:10now remember semi-major axis cubed P
1:13squared orbital period squared we plug
1:16in our numbers 1.5 and 2 and then we
1:19just have to punch this in the
1:20calculator and now you want to be a
1:21little careful putting this into your
1:23calculator actually because what a lot
1:25of people might do is 1.5 cubed divided
1:28by 2 and you have to be a little careful
1:30because you might do divide by 2 and
1:33then square it and you're gonna get it
1:34wrong you want to make sure to square 2
1:36or in other words for but you want to
1:39make sure you might want to use brackets
1:41within your calculator to make sure this
1:42works out ok so be a little careful in
1:47my case if I'm using the calculator on
1:49the computer here you can see I do 1.5 I
1:53cube that and then divide 2 and this I
1:58don't want to say equal yet I have to
1:59make sure that gets squared and now I'm
2:02gonna do that so be a little careful and
2:04try this out on your calculator make
2:06sure to go try this make sure you get
2:08the right answer because it's easy to
2:10make a slip but sure enough we should
2:11get 0.8 for
2:13so the mass of this dissing star is
2:15about 84% the mass of our Sun or point
2:19eight four times the mass of our Sun so
2:21that's how we do this kind of problem
2:23and your book has another example by the
2:26way if you need another example and you
2:28might want to try doing some numbers
2:29yourself too