Good good study, day day up
Section1: Coordinate system
Section2: The Milky Way Galaxy (p. 1044)
First, the structure of the Galaxy.
Face-on: usually no jet.
Pole-on: view along with the jet.
the difference of face and pole on depends on the object and celestial.
Alpha Centauri---有大项目计划send explorer去
We are in Globular Cluster? right?
Galactic coordinate 银道坐标
Use the Galactic structure as a reference frame.
The distance of solar system to GC is about 8.0 kpc --> Galactocentric distance (银心距)
Galactic longitude 银经 (l)
Galactic latitude 银纬 (b)
- unit usually use degree.
The direction of th Galactic rotation (银河自转)
由北pole向下face on，顺时针旋转，一周的时间约250 million yr. (Clockwise as we see from the North Pole)
What is Stellar halo(恒星晕)?
Conversion between galactic and equatorial coordinates
--> Just change the direction of the axis:
The Galactic Center (G.C.) in J2000.0 G.C. is at , for equatorial system are:
- HEASARC Coordinate Converter
Note: angular distance on a celestial sphere
Section3: Conclusion of the last class and so on.. --> Time systems used in astronomy
what is sidereal time? -->恒星时
G.C. : Sgr
Q: What is the angular separation of the closest possible double star?
- This depends on the angular resolution of the telescope
- Two main factors determing the angular resolution of the telescope (p. 168)
- Seeing (视宁度): Sgaroness of a telescopic image. Seeing depends on the degree of turbulence in the Earth's atmosphere at telescope sites. ~0.1'' in the best case (in the case of optical)
- Diffraction limit (衍射极限): Theoretical limit of angular resolution due to wave nature of light. , more detail plz see here
Radial velocity : 径向速度(视向速度)
Transverse velosity : 切向速度
Additional note on transitions
- Electronic transition (电子跃迁)
- Energy: optical, ultraviolet, shorter wavelength
- Vibrational transition (振动跃迁)
- Energy (hv): intrared
- Rotation transition (转动跃迁)
- Energy: Radio
- Hyperfine transition (超精细跃迁)
- Energy: Radio (very long )
---- Good question on the beginning😀
Solar time (太阳时)
- The time sytem based on the Solar position
- Noon (12pm) in the Solar time is
- the annually averaged time of the Solar meridian passage
- Note: meridian passage = 中天
Sidereal time (恒星时)
- The time system based on the positions of stars
- Noon (12pm) ....
Common year 平年 --- 1/4 day shorts of a complete orbit
Leap year 闰年 +3/4 day over a complete orbit
Tropical year 回归年 ---One complete orbit around the Sun
Local standard time (LST)
Universal time (UT, GMT)
-The mean solar time at the Greenwich meridian(LONGITUDE = )
Julian day (儒略日)
Time-variation plot with date (年月日, i.e., Gregorian calendar) on horizontal axis
---not ez for observer
Why dont use 年月日？
- Numerical analysis is not ez
- Difficult to read the number of days
- Leap year
So in astronomy, we use Julian day
What is Julian day?
- Julian day (JD) is the total number of days from JD 0.0
- JD 0.0 is defined as
- the noon (UT) on Jan, B.C. 4713, why Noon??
- bcz Day doesnt change in the night
- Noon (UT) on 1st Jan 2000 is JD 2,451,545.0
- 6pm (UT) on 1st Jan 2000 is JD 2,451,454,25
- Today now is (Sep 2021, 11h40m0s CST) JD 2459476.31944
Modified Julian Day (MJD) MJD=JD-2400000.5
Note: MJD starts at midnight
Heliocentric(以太阳为中心的) Julian Day (HJD)
Practice a bit
(1) Calculate Julian Day for September 2021 1pm CST
(2) What is the corresponding modified Julian day (MJD)?
Homework next week:
and how to transform from UT to ST?
- 如何转换UT to ST?
Trying to explain :
- What is Solar Time and Sidereal Time (st)?
- Why observatory use ST time?
- How to transform from UT to ST?
在开始之前(Before the beginning)：
Sidereal time 是基于恒星的， 这个恒星不是太阳的意思，而是除太阳外的恒星，或是一些深空天体，比如星云或者类星体等(详情可见here)
Sidereal time bases on STAR, and this "STAR" is not the Sun, but some other stars, or some deep space objects, like Nebula or Quasar (see more detail at here)
一些常用缩写的定义 (Definitions of some frequently used acronyms):
UT: Universal Time (reflect the Solar time, the duration from one solar noon to next)
UTC: Coordinated Universal Time (leap second, subset of UT)
GMT: Greenwich Mean Time (time zone)
CST: China Standard Time (UTC/GMT+8)
- When a celestial object passes the meridian passage point, the time of LST is the RA of the object, its really convenient for astronomer.
- Solar time is based on our Sun, and Sidereal time is based on star, the latter is more collimated, can moderately omit the influence of the Earth revolution.
Please just follow the picture below.
Section3~4 (follow-up 2021.9.22): Distance unites used in astronomy
Julian Day Calculator: https://www.aavso.org/jd-calculator
-->See, 3. The continuous spectrum of light in the textbook
1 tropical year!!!
Astronomical Unit (AU, 天文单位)
Averaged distance between the Sun and the Earth
Fundamental of the distance unit used in astronomy
Trigonometry (三角测量) is the distance measurement method using the trigonometric parallax (三角视差)
Concepts: B: base line (基线), p: parallax angle (视差角)
Trigonometry in Space
In priciple, we can measure the distance to stars with trigonometry, but stars are very far away.
So, we need a long baseline to see the parallax angle.
Definition of parsec
Parallax angle is very small,
So, the formula can be approximated as follows:
We can convert the angle from rad to arcsec as follows:
So, the distance formular can be changed as follows:
Therefore, when p''=206,264.806 [AU}, the
With this definition, the distance formula can be changed as follows:
Summary of parsec
- The distance at which 1 AU subtends (对着) an angle of 1''
- Relation with "light year", 1 [pc]=3.26[ly]
Centauri (The nearest star to us) ~ 1.3 [pc]
Relation between parallax angle and parsec
The relationship between parallax angle (p'') and distance (d) is beautifully obcvious.
Exercise a bit
so we have, =6039.43351968km
Nakashima's answer: annular parallax angle is
Little small discussion:
How to measure 1 AU?
2021.9.26-27 Continue at last lecture....
Brief introduction of Maser:
Process: pumping radiation(泵浦辐射，水泵现象)，造成布居数反转。
cm is used due to the good reflection rate.
cm used to investigate the Venus atmosphere
Laser Ranging (激光测距) 而是 Radar Echo (雷达回波)
Scale height H:
z: distance from the disk
: number density of stars at z=0
P: number density of stars at z
Continue on ..
Elongation(距角): The angular distance(这是一个角度，而不是距离) separating the Moon or a planet from the Sun
Venus at the greatest eastern elongation (东大距)----evening
At the greatest western elongation (西大距) ---- in the morning
How to obtain the Galactic rotation curve from gas?
We can derive the Galactocentric distance and radial velocity of the gas component at tangential point D.
VLBI --- 限于看小angular distance，高亮的源，可参考Fourier Transform.
Section 5: The Magnitude Scale
(see, "3. The Continuous Spectrum of Light" in the textbook)
Apparent magnitude (视星等)
--> Basic idea formed in ancient Greek
(1 mag difference (m=1))=(a factor 2.512 times in brightness)
for instance, m=2, factor is ....
How many times different above two magnitudes in brightness?
import numpy as np L=3.839*10**26 r=1.496*10**11 S=4*np.pi*r**2 flux=L/S print(flux)
Qualitative introduction of radio interferomerry EHT
- Short baselines are sensitive to large stucture
- Long base lines are sensitive to small structure
THe interferometer is mathematically the Fourier transform.
Limited number of antennas cannot completely reproduce the source structure
二维傅立叶变换，反映的是梯度Gradient! --> 空间频率+相位图
- The true emission should not change in shape if we use different CLEAN box sizes.
1 Jy =
m-M is mathematically equivalent to the distance:
so far, we have assumed that there is no interstellar extinction!!
Interstellar extinction (星际消光)
Here we define interstellar extinction :
interstellar extinction depends on the wavelength! 河内和外都会有，主要集中在光学波段
Some more details on radio interferometry
visibility=brightness fringe pattern
point spread function: 点扩散函数
- CLEAN is the dominant deconvolution algorithm in radio astronomy.
- Identify the highest peak in the dirty map as a point source.
- Subtract a fraction of this peak from the residual map using a scaled dirty beam
- Add this point source location and amplitude to Clean Component list (https://2199k.cn/post-images/1633922962928.JPG)
- Go to step 1 (an iteration) unless stopping criterion reached
- Stopping criteria:
- Residual map maximum < threshold =several X rms
References for further study of radio astronomy.
- Essential Radio Astronomy
- Interferometry and Synthesis in Radio Astronomy
- NRAO Systhesis ...
- CASA ...
Black body radiation
Neutrinos doenst belong to** Cosmic rays**---> because Cosmic rays are electrul charged.
where is effective temperature.
Plank's law 波长到频率的转换参考:
- If we know the equation , then we can:
So, in the case , this form becomes like:
so we have finally:
Rayleigh-Jeans --> 右边
Wien --> 左边
Effective temperature: T calculated under the assumption of black body radiation.
Note that effective temperature not always represent the True temperrature.
Maser pumping mechanism
- Collisional pumping
- Collision with ambient molecules
- This often happens in interaction regions between two kinematic componnets
- Radiative pumping
- Radiation from a star etc. (SiO maser in evoleved starts)
- Line over lapping
Photometric bands and color index #1
Section 7, (see, "3. The continuous spectrum of light" in textbook)
Photometry and photometric band
In the reality, the usually the intensity is measured in a certain range of .
The wavelength ranges used for photometry are called photometric bands
We use a "filter" to limit the wavelength range.
Filter can pass the radiation only a certain range of .
Johnson-Cousin UBVRI system (3000-9000):
- CCD arrays enable us to do imaging and photometry at one time
The apparent and absolution magnitudes can be defined for each photometric band.
U, B, V can be measured separately
: passband characteristics of the U band filter.
The difference in magnitude like U-B and B-V is called "color index"
As we see above, color is independent of distance !!!
The color index U-B equals to a ratio of flux values at U band B-bands:
So, the color index uniquely determines the effective temperature.
--> color is relying on effective temperature
Color index #2
HR (Hertzsprung-Russell) diagram.
对于O型恒星，很多电子跑路了，所以spectral feature is weak.
- Definition of the bolometric correction
- Meaning of the bolometric correction
- Possibly, could be a useful tool to obtain the total energy emitted from source
- Limitation of the bolometric correction
The bolometric correction BC is defined as follows:
: apparent bolometric magnitude
: apparent V-band magnitude
"bolometric" means "total energy" over "all" wavelength
Meaning of the bolometric correction
If we know BC, we can convert to
For instance, for bright(nearby) stars, and V are measure, and BC is determined.
Then, the obtained BC is applied for similar stars, which possibly show the similar SEDs.
BC always taks a negative value, because flux for is larger than that for
Limitation of the bolometric correction
- It is often impossible to properly measure , because the photometric system doesnt cover entire SED.
- SEDs are not completely the same even among the stars with the same spectral type.
"color" is altered by extinction, and this effect on color is called "color excess" or "reddening"
The color excess isimportant from 2 viewpoints:
- Calculating the distance of a certain object
- Investigating the properties of interstellar materials.
The color excess isquantitively defined as follows:
where is not minus, but a funtion !!
Using , the above formula can be written as follows:
is in proportional to :
The procedure to derive :
Prove that specific intensity is independent of distance:
心得: 需要注意的是，像 这种量，放在积分里面才会有其"单位"立体角的数学物理意义，单独放在外面不能简单认为是1sr里面的Intensity
Defining flux and luminosity
- Section 12, Based on Section 2.1 in ERA(Essential Radio Astronomy)
Specific flux=Flux density=Spectral flux
1 [Jy]= 
通常用于Structure is resolved;
通常用于Structure is not resolved.
Optical depth and Radiative trasfer equation
- Section 13, Based on Section 2.2 in ERA(Essential Radio Astronomy)
--> linear absorption coefficient
Only consider absorption:
Optical depth :
Contribution from emission:
: emission coefficient
Intro to thermal equilibrium (TE)
Under TE, is the black body radiation:
The Planck function does not depend on material, so under TE, the dependency on material is cancelled out.
Simple applications of radiative transfer:
(Based on Section 2.2.3 in ERA)
When , the black body radiation is approximated by the Rayleigh-Jeans law,
Then the temperature T is calculated to be :
Even if we replace with mathematically T can be calculated:
is called "brightness temperature"
not a real physical temperature, but often the indicator of
The sum of all noises, which is converted to the temperature scale [K], is called the system temperature ()
Observation to determine
Emission or absorption
if , emission line is formed
if , absorption line is formed
Line radiative transfer fundamental
Section 17, see e.g., ERA 7.3
Q&M: How can we describe the microscopic phycsical processes in the equation of RT?
- Profile function:
- Radiation energy corresponding to the line:
- Einstein coefficients:
***Only need to know one Einstein coefficient, and another two can be derived.
Defination: T dericed from Boltzman equation, not real temperature.
Q: what is Boltzman distribution?
point at LTE
See. e.g., ERA 2.3