PHYS 1403 Lab 7: APPARENT & ABSOLUTE VISUAL MAGNITUDE
Worksheet
Name:
CWID:
OBJECTIVE
You will learn about apparent and absolute visual magnitudes and see how
astronomers use these terms in their work.
EQUIPMENT
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PHYS 1403 Lab 7: APPARENT & ABSOLUTE VISUAL MAGNITUDE
Worksheet
Name:
CWID:
OBJECTIVE
You will learn about apparent and absolute visual magnitudes and see how
astronomers use these terms in their work.
EQUIPMENT
Calculator, Stellarium software, a digital (or cell phone) camera.
INTRODUCTION
The apparent visual magnitude of a star is a brightness scale established by
early astronomers. It's since been refined by modern day astronomers and continues to
be used even in professional research. It is an inverted scale… the smaller the apparent
visual magnitude (mv), the brighter the star (with negative magnitudes indicating a
brighter star than positive magnitudes). For example, a very faint star might have an mv
of 5.0 whereas a bright star might have an mv of -1.0. The human eye can detect faint
stars with an mv of 6.0 assuming you're out in the country far away from city lights and
the Moon is not visible. A typical amateur telescope can detect stars with an mv of 13.0.
Large professional telescopes can detect stars with an mv of 22.0.
According to the tables attached to this lab, Barnard's star has an mv of 9.54.
This means you cannot see this star with the naked eye even in the best of viewing
conditions but you can easily see it with an amateur telescope. According to the same
table the star 61 Cygni has an mv of 5.22. This means you can probably see this star
with the naked eye but it's near the limit. You'll probably have to go out in the country to
see it.
When modern day astronomers refined the apparent visual magnitude scale for
professional use they introduced negative values for mv. For example, the tables
attached to this lab indicate that the star Sirius has an mv of -1.46. Compared to Luyten
789-6 with an mv of 12.18, Sirius is very bright. Luyten 789-6 requires an amateur
telescope to observe whereas Sirius is easily visible to the naked eye even in a large
city or during a moonlit sky - in fact, Sirius is the brightest star in the night sky.
The Sun has an mv of -26.72. This makes the Sun much brighter than Sirius
from our perspective on Earth. But Sirius is 8.6 light-years from us, meaning it takes
light 8.6 years to travel from Sirius to Earth
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