import numpy as np

Make some arrays that hold star temp and mass

# set up arrays holding temperature and sizes of some stars
temp = np.array([  3000,   8000,  6000,  4000, 9000,  3500,  7000,  10000])
mass = np.array([ 0.514,  2.015,  1.05,  0.75,  2.5,  2.03,  1.51,   0.50])

Define a boolean flag that defines hot stars to be hotter than the Sun

Tsun = 5770
hot = temp > Tsun
print(temp)
print(hot)

[ 3000  8000  6000  4000  9000  3500  7000 10000]
[False  True  True False  True False  True  True]

Work out how many stars there are, and how many of them are hot

# len() tells you how many elements are in an array
print('There are {} stars'.format(len(temp)))

# but you can also sum a boolean array to figure out how many satisfy the condition
print('There are {} hot stars'.format(np.sum(hot)))

There are 8 stars
There are 5 hot stars

Work out average mass of all stars, and average mass of just the hot stars

# this averages all the star masses in the array
avmass_all = np.average(mass)
# this averages the star mass of just the stars with hot temps: temp[hot]
avmass_hot = np.average(mass[hot])

# print out the average mass, notice how I make sure not to print out tons of digits!
print('The average mass of ALL stars is {:.2f} Msun'.format(avmass_all))
print('The average mass of HOT stars is {:.2f} Msun'.format(avmass_hot))

# if I had omitted the ':.2f' it would have looked terrible:
print('The average mass of HOT stars is {} Msun  <--- yuck!'.format(avmass_hot))

The average mass of ALL stars is 1.36 Msun
The average mass of HOT stars is 1.52 Msun
The average mass of HOT stars is 1.5150000000000001 Msun  <--- yuck!

Figure out which stars are more massive than the Sun

bigmass = mass > 1
print(bigmass)
print('There are {} massive stars'.format(np.sum(bigmass)))

[False  True  True False  True  True  True False]
There are 5 massive stars

Stacking boolean flags

# a hot main sequence star should be hotter than the Sun and more massive than the Sun
hot_ms = np.logical_and(hot, bigmass)
print('There are {} hot main sequence stars'.format(np.sum(hot_ms)))

There are 4 hot main sequence stars

Using the '~' modifier

# with a boolean flag, a '~' in front means 'not'
# so white dwarfs are stars that are hot but that are not big mass
wd = np.logical_and(hot, ~bigmass)

print('There are {} white dwarf stars'.format(np.sum(wd)))

There are 1 white dwarf stars