Astronomy 2110 – Intro. to Astrophysics I

• Mike Skrutskie – mfs4n – 924-4328

• Room 262 – Astronomy Building

– Office Hours (still being rearranged):

• Monday 1:30 – 2:30

• Thursday 11:00 – 12:00

if those don't work contact me for arrangements

• Course TAs

– Connor McClellan (bcm2vn) and Mark Siebert (mas5fb)

• Room 267/ Astronomy Building

• Friday discussion and by appointment

•

http://faculty.virginia.edu/skrutskie/ASTR2110/

Mike

Fan Mountain Infrared Camera Telescopes are only as good as the instruments that collect light

attached to the back end.

This infrared camera was designed and fabricated from scratch by UVa graduate and undergraduate students.

Sloan Digital Sky Survey - APOGEE

60” x 60”

5” x 5”

Image Reconstructon

Loki Resolved

Europa Occults Io

The Wide-Field Infrared Survey Explorer• “WISE” has mapped the entire

sky at infrared wavelengths

– 3, 5, 12, and 23 micrometers to be specific.

• Solid hydrogen cooled the detectors and telescope to temperatures as low as 8 degrees above absolute zero.

• WISE was launched from Vandenberg Air Force Base on a Delta-2 rocket on December 14, 2009.

– WISE is in a 500 km high orbit that keeps it continually over the sunrise/sunset line.

New Horizons and 2014MU69

A Few Words About Accuracy and Precision

- Accuracy: Closeness to “Truth”

- Precision: “Tightness” of the sample distribution and/or number of appropriate decimal places.

A Few Words About Accuracy and Precision

- In this class… only enough precision to convey an approximately accurate answer… you’ll see what I mean…

- If you’re writing down more than 2 significant digits you had better have a very good reason.

A Few Words About Accuracy and Precision

- Why do I really care….Excessive precision leads to a myopic perspective that cranking through equations is what counts.

Comfort (and skill) with Approximation

Developing a facility with obtaining quick (imprecise and somewhat inaccurate) answers is the gateway to true understanding.

It’s not about the equations….

Comfort (and skill) with Approximation

Developing a facility with obtaining quick (imprecise and somewhat inaccurate) answers is the gateway to true understanding.

It’s not about the equations….

How much does the Washington Monument weigh…. on Pluto

First day, I’d tell the class all I knew,” he told The Saturday Evening Post, “and there was nothin’ left to say for the rest o’ the semester.” - Andy Griffith on teaching

Discussion section (today only) will be held in Astronomy Building Room 201

(as opposed to the regular site, Gilmer 166)

Also consider the opportunity to get involved with the Lucy mission to the Jupiter Trojans.

https://lspace.asu.edu/

Approximating an Answer

● How much does the Washington Monument weigh?● Mass is volume times density

● The density of water is 1000 kg/m3 (1 g/cm3)● Stone is about 3x greater● But the monument is hollow… use 1000 since it is a round number

● 10m x 10m x 100m is a pretty good approximation to dimensions (in fact a little low, which is good because the density used is a bit high – great way to trim up the answer.

● 104 cubic meters x 103 kg/m3 = 107 kg●

● Scientific notation is your friend and primary tool (this is a logarithmic exercise).

● There are no wrong answers, only poor strategies….

Surface Gravity on Pluto

● Weight is the force of gravity on an object, the product of the acceleration of gravity times the object’s mass.

● Quantitatively:

● Returning to our friend, density – a reliably estimated number,

ag=GM planet

R2

M planet=43

π R3ρ

ag=4 π G Rρ

3

Surface Gravity on Pluto

● Weight is the force of gravity on an object, the product of the acceleration of gravity times the object’s mass.

● Quantitatively:

● Returning to our friend, density – a reliably estimated number,

ag=GM planet

R2

M planet=43

π R3ρ

ag=4 π G Rρ

3

Since we are scaling from the weight on Earth, we really just need to know how things change going to Pluto.

The only things that change are R and ρ. Gravitational acceleration changes directly in proportion to both. Halve R, halve the acceleration.

Orbital Velocity in the Solar System

• You're set for life if you remember that the Earth orbits the Sun at a speed of 30 km/s.

Where R is expressed in astronomical units

vorbital = √GM sun

R

vorbital = 30 km / s √1RA.U

Orbital Velocity in the Solar System(s)• Derivation of this scaling ... basically multiply by “1” at various

locations and separate out the equation for Earth's orbital speed (the 30 km/s).

• Mo is the mass of the Sun. RAU is the Earth's distance from the Sun.

where R is expressed in astronomical units

30 km/s Distance from star in astronomical units

Mass of the star compared to the Sun

vorbital = √GM

M o

M o

RRAURAU

= √GM o

RAU √M /M o

R /RAU¿

vorbital = 30 km / s √ 1RA.U

Relative scale is also your friend. Jupiter, for example is 1/1000th the mass of the Sun – a milliSun. Jupiter’s diameter is 1/10th that of the Sun’s.

Instead of meters/kilometers, the Astronomical Unit (AU) is a great measuring stick in the Solar System. Jupiter is 5 AU from the Sun. The New Horizons Spacecraft is now 45 AU from the Sun. It encountered Pluto when it was 33 AU from the Sun.

What if You’re Stuck Explicitly Calculating

● Calculator optional!

ag=GM planet

R2

ag=4 π G Rρ

3

How Do You Make Sense of it All?

Astronomy: A Search for Organization?

Stars....

Galaxies....

Astronomy: A Search for Organization?

Astronomy: A Search for Structure?

Hierarchical structure in the Universe leads to an extended mailing address.....

Earth

The Solar System (in capital letters)

The Milky Way Galaxy

The Local Group

The Local Supercluster

The Universe

Implicit in understanding this structure is knowing how far away things actually are.... not an easy task!

Understanding Structure: Divining Depth

Understanding Structure: Divining Depth

Astronomy: A Search for Origins!!!

Where did all the structure and organization come from?

How did it emerge over time?

What were the starting conditions?

What physical processes were important?

How have things changed over cosmic time?

We wish to explain, not just observe.

Astronomy: A Search for Origins!!!

Where did all the structure and organization come from?

How did it emerge over time?

What physics/physical processes were important?

Hydrogen + Helium + Gravity Stars, Galaxies, and ⇨ Humans

given about 14 billion years.

Astronomy: A Search for Origins!!!

Where did all the structure and organization come from?

How did it emerge over time?

What physics/physical processes were important?

Hydrogen + Helium + Gravity Stars, Galaxies, and ⇨ Humans

given about 14 billion years.

⇨ = PHYSICS

Evolution of Understanding

Shaking fist at skies in anger and frustration and awe

Evolution of Understanding

Ad hoc explanations of heavenly phenomena based up fanciful beliefs and stories

Evolution of Understanding

Physical, but unrealistic, models of the heavens that have predictive power

Evolution of Understanding

Simple mathematically-based physical models and laws (physics) that provide an accurate context for astronomical understanding.CS

a=Fm

Evolution of Understanding

Powerful mathematical/physics formalisim combined with extensive observational and computational capability enabling precise modeling and realistic global simulation.CS

Evolution of Understanding

In

- Intuitive mastery of physical law.

- Understanding through insight. ● Developing a “feel” for problems and problem solving

- Ability to scale solutions independent of rote calculation. ● Developing intuition for “the answer”.

Channel Your Inner Feynman

In

- Intuitive mastery of physical law.

- Understanding through insight. ● Developing a “feel” for problems and problem solving

- Ability to scale solutions independent of rote calculation. ● Developing intuition for “the answer”.

How Can it be Practical to Deliver Your Own Interplanetary Launch Platform to the Surface of

the Moon???

mom p

=eΔ vv exhaustΔ v = vexhaust ln

moriginalm payload

Escape Velocity

• Make an object's energy positive.

• That is, kinetic energy exceeds gravitational potential/binding energy.

Equating the two gives you the “escape velocity”

•

•

•

• Escape velocity from the surface is sqrt(2) larger than orbital velocity.

K.E. =12mv2 P.E. =

GMmR

vescape = √ 2GMR

mom p

=eΔ vv exhaustΔ v = vexhaust ln

moriginalm payload

Earth escape velocity = 11.2 km/s

Lunar escape velocity

= 2.4 km/s

Typical (chemical) rocket nozzle exhaust velocities are 2-3 km/s

Ryugu (Hyabusa2) Bennu (OSIRIS-REx)

https://science.sciencemag.org/content/sci/365/6455/817.full.pdf https://science.sciencemag.org/content/sci/365/6455/817.full.pdf

sampling