Lecture 1: The Milky Way galaxy
A
B
A is what we see from Earth inside the Milky Way while B is what the Milky Way “might” look like if we were far away looking back at our own galaxy from some other galaxy.
Which is a picture of the Milky Way?
ﺎﻨﺗﺮﺠﻤﻟ ﺔﻴﺨﻳرﺎﺗ ةﺮﻈﻧ
” ﺔﻧﺎﺒﺘﻟا برد
“
• ﺔﻨﻳﺪﻤﻟا ءاﻮﺿأ ﻦﻋ ًاﺪﻴﻌﺑو قﺎﺤﻤﻟا ﻲﻟﺎﻴﻟ ﻲﻓ ًﺎﻌﺻﺮﻣ ءﻮﻀﻟا ﻦﻣ ﺎﺘهﺎﺑ ﺎﻄﻳﺮﺷ ىﺮﺗ نأ ﻊﻴﻄﺘﺴﺗ لﺎﻤﺸﻟا ﻩﺎﺠﺗا ﻦﻣ ءﺎﻤﺴﻟا ﺮﺒﻋ ﺪﺘﻤﻳ مﻮﺠﻨﻟﺎﺑ
ﻲﺑﺮﻐﻟا بﻮﻨﺠﻟا ﻰﻟإ ﻲﻗﺮﺸﻟا .
• ﻦﺒﻟ ﻪﻧﺄآو ﻂﻳﺮﺸﻟا اﺬه ﻢﻬﻟ اﺪﺑ ﻖﻳﺮﻏﻹا ءﺎﻣﺪﻗ ﻖﻳﺮﻄﻟا ﻢﺳا ﻪﻴﻠﻋ اﻮﻘﻠﻃﺄﻓ ءﺎﻤﺴﻟا ﻲﻓ ﺐﻜﺴﻧا
ﻲﻨﺒﻠﻟا .
• برد نأ ﻮﻴﻠﻴﻟﺎﺟ ﻦهﺮﺑ ﺮﺸﻋ ﻊﺑﺎﺴﻟا نﺮﻘﻟا ﻲﻓ ﺔﺟرد ﻰﻟإ ﻪﻤﺗﺎﻌﻟا مﻮﺠﻨﻟا ﻦﻴﻳﻼﻣ ﻦﻣ ﺔﻔﻟﺆﻣ ﺔﻧﺎﺒﺘﻟا
ﺎﻬﺘﻣﺎﺘﻌﻟ ءﻮﻀﻟا ﻦﻣ ﺔﻠﺼﻔﻨﻣ ﺎﻃﺎﻘﻧ ﺎﻬﺘﻳؤر ﺢﻴﺘﺗ ﻻ .
ًﺎﺻﺮﻗ ﺎﻨﺴﻤﺷ ﻊﻣ نﻮﻜﺗ مﻮﺠﻨﻟا ﻩﺬه نأ ﻢﻠﻌﻧ مﻮﻴﻟاو ﺎﻨﺗﺮﺠﻣ ﻮه ،ءﻂﺒﺑ روﺪﻳ ًﻼﺋﺎه .
•
ﺔﻧﺎﺒﺘﻟا برد ةﺮﺠﻣ فﺎﺸﺘآا :
-
ﺔﻣﻮﻈﻨﻣ ﺎﻬﻧأ ﻰﻠﻋ ﺔﻧﺎﺒﺘﻟا برﺪﺑ ﺎﻨﺘﻓﺮﻌﻣ ﻰﻗﺮﺗ رﺎﺷأ ﺎﻣﺪﻨﻋ ﺮﺸﻋ ﻦﻣﺎﺜﻟا نﺮﻘﻟا ﻰﻟإ ﺔﻴﻤﺠﻧ ﻲﻧﺎﻤﻟﻷا فﻮﺴﻠﻴﻔﻟاو ﺖﻳار سﺎﻣﻮﺗ ﻦﻴﻴﻜﻠﻔﻟا ﺔﻧﺎﺒﺘﻟا برد نأ ،ﻩﺪﺣ ﻰﻠﻋ ﻞآ ،ﺖﻧﺎآ ﻞﻳﻮﻧﺎﻤﻳإ مﻮﺠﻨﻟا ﻦﻣ ﺔﺤﻄﺴﻣ ةﺮﻬﻤﺟ .
مﺎﻈﻨﻟا نﺎآ ﻮﻟ ﻻﺎﻗو
مﻮﺠﻨﻟا ﻦﻣ ﺔﻳوﺮآ ﺔﺑﺎﺤﺳ ﺰآﺮﻣ بﺮﻗ ﻲﺴﻤﺸﻟا ﻊﻴﻤﺟ ﻲﻓ مﻮﺠﻨﻟا ﻦﻣ ﻪﺴﻔﻧ دﺪﻌﻟا ﺎﻨﻳأﺮﻟ
ًﺎﺒﻳﺮﻘﺗ تﺎهﺎﺠﺗﻻا .
ﻚﻟذ ﻦﻣ ًﻻﺪﺑ ىﺮﻧ ﺎﻨﻧأ ﺪﻴﺑ
ﻪﻨﻣ ﺔﻧﺎﺒﺘﻟا برد ﻩﺎﺠﺗا ﻲﻓ ﺮﺒآأ مﻮﺠﻨﻟا ﻦﻣ ًادﺪﻋ
ًﺎﻣﺎﻤﺗ ﻪﻌﻗﻮﺘﻧ ﺎﻣ اﺬهو ﻪﻴﻠﻋ ﺔﻳدﻮﻤﻋ تﺎهﺎﺠﺗا ﻲﻓ
مﻮﺠﻨﻟا ﻦﻣ ةﺮآ ﻻ ًﺎﺻﺮﻗ ﺎﻨﺗﺮﺠﻣ ﺖﻧﺎآ اذإ
.
•
مﺎﻴﻠﻳو مﺎﻗ ﺮﺸﻋ ﻦﻣﺎﺜﻟا نﺮﻘﻟا ﺔﻳﺎﻬﻧ ﻲﻓ ﻞﺷﺮه
دﺪﻋ ﻰﺼﺣأ ﺎﻣﺪﻨﻋ رﺎﻜﻓﻷا ﻩﺬه ﻞﻘﺼﺑ
ﻔﻳ صﺮﻘﻟا ضﺮﻋ ﻚﻟذ ﻦﻣ ﺞﺘﻨﺘﺳاو ﺔﻔﻠﺘﺨﻤﻟا تﺎهﺎﺠﺗﻻا ﻲﻓ ﺔﻴﺋﺮﻤﻟا مﻮﺠﻨﻟا ﺔﻜﻤﺳ قﻮ
تاﺮﻣ ﺲﻤﺧ .
صﺮﻘﻟا لﻮﺣ تﺎهﺎﺠﺗﻻا ﻞآ ﻲﻓ ﺎﺒﻳﺮﻘﺗ وﺎﺴﺘﻣ مﻮﺠﻨﻟا دﺪﻋ نأ ﺪﺟو ﺎﻤﻟو
ﺎﺒﻳﺮﻘﺗ ﻪﻄﺳو ﻲﻓ ﻊﻘﺗ ﺎﻬﻧأ ﺪﺑﻻ ﺲﻤﺸﻟا نأ ﻰﻟإ ﺺﻠﺧ .
دﺎﻌﺑأ سﺎﻴﻘﻟ ﻞﻴﺒﺳ لإ ﺪﺘﻬﻳ ﻢﻟ ﻪﻧﻷو
ﺔﻧﺎﺒﺘﻟا برد ﻢﺠﺣ ﺪﻳﺪﺤﺗ ﻊﻄﺘﺴﻳ ﻢﻠﻓ مﻮﺠﻨﻟا
.
• ﺪﻳ ﻰﻠﻋ ﻦﻳﺮﺸﻌﻟا نﺮﻘﻟا ﺔﻳاﺪﺑ ﻲﻓ ةﺮﻣ لوﻷ ﺔﻧﺎﺒﺘﻟا برد ﻢﺠﺣ سﺎﻴﻗ ىﺮﺟ يﺪﻨﻟﻮﻬﻟا ﻦﻴﻴﻜﻠﻔﻟا
سﻮﺑﻮﻜﻴﺟ ﻲﻜﻳﺮﻣﻷاو ﻦﻴﺘﺑﺎآ
ﻮﻟرﺎه ﻲﻠﺑﺎﺷ
. ﺔﻘﻳﺮﻄﺑ ﺔﺒﻳﺮﻘﻟا مﻮﺠﻨﻟا ﻰﻟإ ﺔﻓﺎﺴﻤﻟا ﻦﻴﺘﺑﺎآ سﺎﻗ
ﺮﻈﻨﻤﻟا فﻼﺘﺧا )
parallax (
ﺎﻬﺘآﺮﺣ لﻼﺧ ﻦﻣ ًاﺪﻌﺑ ﺮﺜآﻷا مﻮﺠﻨﻠﻟ ﺔﻓﺎﺴﻤﻟا رﺪﻗ ﻢﺛ ، )
proper
motion (
ﺎهﺮﻄﻗ ﻞﻜﺸﻟا ﺔﻴﺻﺮﻗ ﺔﻣﻮﻈﻨﻣ ﺔﻧﺎﺒﺘﻟا برد نأ ﻰﻟإ ﻰﻬﺘﻧاو 20.000
ﻚﺳرﺎﺑ ﻊﻘﺗ
ﺎهﺰآﺮﻣ بﺮﻗ ﺲﻤﺸﻟا .
• ﺮﺸﻧو ﻻإ ﺔﻧﺎﺒﺘﻟا برﺪﻟ ﻪﺟذﻮﻤﻧ ﺮﺸﻨﻳ ﻦﻴﺘﺑﺎآ ﺪﻜﻳ ﻢﻟ ﻲﻠﺑﺎﺷ
ًﺎﻣﺎﻤﺗ ﻪﻟ ﺎﺿرﺎﻌﻣ ﺮﺧﺁ ﺎﺟذﻮﻤﻧ .
ﺖﻧﺎآو
ﺔﺠﺣ ﻲﻠﺑﺎﺷ
ﺰآﺮﻤﻟا بﺮﻗ ﺖﺴﻴﻟ ﺲﻤﺸﻟا نأو ﻦﻴﺘﺑﺎآ ﺪﻘﺘﻋأ ﺎﻤﻣ ﺮﺒآأ ﺔﻧﺎﺒﺘﻟا برد نأ ﺪﻌﺘﺒﺗ ﺎﻤﻧإو
ﻩﺮﻄﻗ ﻒﺼﻧ ﻲﺜﻠﺛ راﺪﻘﻤﺑ ﻪﻨﻋ .
ﻞﺻﻮﺗ ﺪﻗو ﻲﻠﺑﺎﺷ
ﺔﻳوﺮﻜﻟا دﻮﺸﺤﻟا ﺔﺳارد لﻼﺧ ﻦﻣ ﻪﺠﺋﺎﺘﻧ ﻰﻟإ .
• ﻢﺳﺮﻳ ﻲﻜﻟو ﻲﻠﺑﺎﺷ
ا ﺪﻗو ﺲﻤﺸﻟا ﻦﻋ دﻮﺸﺤﻟا ﻩﺬه ﺪﻌُﺑ ﺔﻓﺮﻌﻣ ﻰﻟإ ﺔﺟﺎﺤﺑ نﺎآ دﻮﺸﺤﻟا ﺔﻄﻳﺮﺧ نﺎﻌﺘﺳ
ﻞﻤﻌﺑ ﻚﻟذ ﻲﻓ ﺎﺘﻳﺮﻨه
ﺖﻴﻔﻴﻟ ﻊﻣ عﻮﻄﺴﻟا ﻲﻓ ﺮﻴﻐﺘﻟا ﺔﻗﻼﻋ تﺪﺟوأ درﺎﻓرﺎه ﻦﻣ ﻚﻠﻓ ﺔﻤﻟﺎﻋ ﻲهو
ةﺮﻴﻐﺘﻤﻟا مﻮﺠﻨﻠﻟ ﻦﻣﺰﻟا .
ﻲﺴﻜﻌﻟا ﻊﻴﺑﺮﺘﻟا نﻮﻧﺎﻗ لﻼﺧ ﻦﻣو يﺮهﺎﻈﻟا ﺎﻬﻧﺎﻌﻤﻟو ﺎﻬﻋﻮﻄﺳ ﺔﻓﺮﻌﻤﺑو
ﻦﻜﻤﺗ ﻲﻠﺑﺎﺷ
ﺬه ﺪﻌﺑ ﻲﻟﺎﺘﻟﺎﺑو ﺔﻳوﺮﻜﻟا دﻮﺸﺤﻟا ﻞﺧاد ةﺮﻴﻐﺘﻤﻟا مﻮﺠﻨﻟا ﺪﻌﺑ بﺎﺴﺣ ﻦﻣ ﻢﺛ ،دﻮﺸﺤﻟا ﻩ
ﺔﻧﺎﺒﺘﻟا برد ﻲﻓ ﺪﺸﺣ ﻞآ ﻊﻗﻮﻣ ًﺎﻴﻧﺎﻴﺑ ﻞﺜﻣ .
ﻢﺠﺣ ﺮﺒﻋ ﺔﻋزﻮﻣ دﻮﺸﺤﻟا نأ ﻪﺗﺎﻄﻄﺨﻣ تﺮﻬﻇأ ﺪﻗو
ﻩﺮﻄﻗ ﺢﻄﺴﻣ ﻲﻧاوﺮآ 100
ﻮﻠﻴآ ﻚﺳرﺎﺑ
ﺪﻌﺑ ﻰﻠﻋ ﻪﻴﻓ ﺲﻤﺸﻟا ﻊﻘﺗ مﻮﺠﻨﻟا ﻦﻣ صﺮﻘﺑ ﻂﻴﺤﺗ ﺎﻬﻧأو
ةﺰآﺮﻣ ﻦﻋ ﺔﻓﺎﺴﻤﻟا ﻲﺜﻠﺛ
Distance measurements to globular clusters define the location of the galactic center.
• Globular clusters form a sphere around the center of the Milky Way .
• In 1917, Harlow Shapley
determined distances to globular clusters by finding variable stars with known absolute magnitude.
• ﺎﻬﻴﻟإ ﻰﻬﺘﻧا ﻲﺘﻟا ﺞﺋﺎﺘﻨﻟا ﻦﻴﺑ ﺮﻴﺒآ قرﺎﻓ ﺔﻤﺛ نﺎآو ﻲﻠﺑﺎﺷ
) ﻊﻗﻮﻣو ﺔﻧﺎﺒﺘﻟا برد ﺮﻄﻗ ﺔﻤﻴﻗ
ﺲﻤﺸﻟا (
ه ﺎﻤآو ،ﻚﻠﻔﻟا ءﺎﻤﻠﻋ ﻦﻴﺑ ﺎﻌﺳاو ًﻻﺪﺟ ﺐﺒﺳ يﺬﻟا ﺮﻣﻷا ﻦﻴﺘﺑﺎآ ﺞﺋﺎﺘﻧو ﻲﻓ لﺎﺤﻟا ﻮ
ًﺎﻴﺋﺰﺟ ﺊﻄﺨﻣو ًﺎﻴﺋﺰﺟ ﺐﻴﺼﻣ ﻦﻴﺒﻧﺎﺠﻟا ﻼآ نﺈﻓ تﺎﻓﻼﺨﻟا ﻩﺬه ﻞﺜﻣ .
ﻦﻴﺘﺑﺎآ جﺎﺘﻨﺘﺳا نﺎآ ًﻼﺜﻤﻓ
فﺎﻔﺷ ءﺎﻀﻔﻟا نأ ضﺮﺘﻓا ﻪﻧﻷ ﻪﻴﻠﻋ ﻮه ﺎﻤﻣ ﺮﻐﺻأ ﺎﻬﻧأ ﺔﻧﺎﺒﺘﻟا برد ﻢﺠﺤﻟ ﺮﺛﻷ ﺮﻈﻨﻳ ﻢﻠﻓ
ﺔﻣﺎﺘﻌﻟا رﺎﺒﻐﻠﻟ
ﻲﻤﺠﻨﻴﺒﻟا .
نﺎآ ﻞﺑﺎﻘﻤﻟﺎﺑو ﻲﻠﺑﺎﺷ
ﻲﻓ ﻎﻟﺎﺑ ﻪﻨﻜﻟو ﺰآﺮﻤﻟﺎﺑ ﺖﺴﻴﻟ ﺲﻤﺸﻟا نﺄﺑ ﺎﻘﺤﻣ
ةﺪﻤﺘﻌﻤﻟا ﺔﻤﻴﻘﻟا ﻦﻣ تاﺮﻣ ثﻼﺛ ﺮﺒآأ ﻪﺘﻤﻴﻘﻓ ﺔﻧﺎﺒﺘﻟا برد ﺮﻄﻗ ﺮﻳﺪﻘﺗ ﺎﻴﻟﺎﺣ
. ﻪﺌﻄﺧ ﺐﺒﺳ دﻮﻌﻳو
تاﺮﻴﻐﺘﻤﻟا ﻦﻣ ﻦﻴﻋﻮﻧ ﻦﻣ ﻞﻣﺎﻌﺘﻳ نﺎآ ﻪﻧأ ﻰﻟإ ﻦﻄﻔﻳ ﻢﻟ ﻪﻧأ ﻰﻟإ ًﺎﻴﺋﺰﺟ ﺔﺿﺎﺒﻨﻟا
مﻮﺠﻧ ﺎﻤه
قﺎﻴﻠﺸﻟا
RR تﺎﻳوﺎﻔﻴﻘﻟاو
. نأ ﻊﻣو ﺖﻴﻔﻴﻟ
عﻮﻄﺴﻟاو روﺪﻟا نﻮﻧﺎﻗ تدﺪﺣ ﺖﻧﺎآ تﺎﻳوﺎﻔﻴﻘﻠﻟ
نﺎآ ،
ﻲﻠﺑﺎﺷ مﻮﺠﻨﺑ ةﺮﺠﻤﻟا ﻢﺠﺣ ﺲﻴﻘﻳ
قﺎﻴﻠﺸﻟا
RR . مﻮﺠﻧ ﺖﻧﺎآ ﺎﻤﻟو
قﺎﻴﻠﺸﻟا ﻦﻣ ًﺎﻋﻮﻄﺳ ﻞﻗأ
تﺎﻳوﺎﻔﻴﻘﻟا ﻦﻇ ﺪﻘﻓ
ﻲﻠﺑﺎﺷ ﻢﺠﺣ ﺮﻳﺪﻘﺗ ﻲﻓ ﻩﻮﻠﻏ ءﺎﺟ ﺎﻨه ﻦﻣو ًًًﻼﻌﻓ ﻪﻴﻠﻋ ﻲه ﺎﻤﻣ ﺪﻌﺑأ ﺎﻬﻧأ
ﻵا ﻪﻴﻠﻋ فرﺎﻌﺘﻤﻟا ﻢﺠﺤﻟا ﻰﻟإ بﺮﻗأ ﻦﻴﺘﺑﺎآ ﺮﻳﺪﻘﺗ نﺎآ ﻦﻴﺣ ﻲﻓ ،ةﺮﺠﻤﻟا ﺎﻤﻴﻓ ﺄﻄﺧأ ﻪﻨﻜﻟو ن
ﺎﻬﻴﻓ ﺲﻤﺸﻟا ﻊﻗﻮﻤﺑ ﻞﺼﺘﻳ .
• جذﻮﻤﻧ ءﺎﻤﻠﻌﻟا ﻊﺟار ﺔﻴﺿﺎﻤﻟا ﺔﻨﺳ ﻦﻴﺴﻤﺨﻟا لﻼﺧ ﻲﻠﺑﺎﺷ
ءﺎﺟ ةﺮﻴﺜآ تﺎﻨﻴﺴﺤﺗ ﻪﻴﻠﻋ اﻮﻠﺧدأو
ﻲه ًﻼﺜﻣ ﺔﻴﺟرﺎﺨﻟا تاﺮﺠﻤﻟا ﻢﻈﻌﻤﻓ ،ىﺮﺧأ تاﺮﺠﻤﻟ تﺎﺳارﺪﻟ ةﺮﻤﺛ ﺎﻬﻤﻈﻌﻣ صاﺮﻗأ
ﺔﺤﺿاو ﺔﻴﺒﻟﻮﻟ عرذﺄﺑ ﺔﺤﻄﺴﻣ .
برد ﻲﻓ ﺎﻨﻌﻗﻮﻣ نﻷ ﺎﻨﺗﺮﺠﻣ ﻲﻓ ﻚﻠﺘآ عرذأ ﻒﺸآ رﺬﻌﺘﻳو
ﺎﻬﺻﺮﻗ ﻞﻣﺎآ ﺔﻳؤﺮﺑ ﺎﻨﻟ ﺢﻤﺴﻳ ﻻ ﺔﻧﺎﺒﺘﻟا .
•
ذ ىﺮﺧﻷا ﺔﻴﺻﺮﻘﻟا تاﺮﺠﻤﻟا نأو ﻂﺴﺒﻨﻣ صﺮﻗ ﺔﻧﺎﺒﺘﻟا برد نأ ﺎﻨﺿﺮﺘﻓا ﻮﻟ ،ﺔﻴﺒﻟﻮﻟ عرذأ تا
ًﺎﻀﻳأ ﺎﻨﺗﺮﺠﻣ ﻲﻓ عرذﻷا ﻚﻠﺗ دﻮﺟو ﺞﺘﻨﺘﺴﻧ نأ ﻲﻘﻄﻨﻤﻟا ﻦﻣ نﺈﻓ .
• ﻦﻣ ﻪﻧﻮﻓﺮﻌﻳ ﺎﻣ ﻰﻟإ ﺎﻨﺗﺮﺠﻣ ﻰﻠﻋ ﻢﻬﺘﺳارد ﺞﺋﺎﺘﻧ نﻮﻴﻜﻠﻔﻟا ﻢﺿ ﺪﻘﻓ ﻚﻟﺬﻟ ﻲﺘﻟا ﺔﻣﺎﻌﻟا ﺮهﺎﻈﻤﻟا
ﻒﺼﺘﺗ ﺎﻬﺑ
ﺔﻧﺎﺒﺘﻟا برﺪﻟ ﻞﺼﻔﻣ جذﻮﻤﻧ ﻊﺿو ﻦﻣ اﻮﻨﻜﻤﺘﻴﻟ ،ىﺮﺧﻷا تاﺮﺠﻤﻟا
The structure of our Galaxy
• A galaxy is a huge collection of stellar and interstellar matter isolated in space and bound together by its own gravity.
• Astronomer study the structure of the Galaxy to infer its past and predict its future.
• Our model is pretty good. It pictures the Galaxy in three main parts: a central region called galactic (nuclear) bulge, a disk, and a halo.
Schematic diagram showing the structure of the Galaxy
Regions of the Milky Way Galaxy
Diameter of disk = 100,000 LY. (30.000 pc)
Number of stars = 400 billion
Thickness of disk = 1.000 LY (300 pc)
Sun is in disk, 28,000 LY. out from center
Other spiral galaxies give a good impression
of the true structure of our own Milky Way system
M51 in Canes Venatici
M31 in Andromeda
Edge-on spiral galaxy, M104. Note the thin but dark dust lane
and the unusually large bulge relative to the disk
Spiral galaxy NGC1232. The redder stars in the central bulge and bluer stars in the spiral arms are typical of
spiral galaxies, including our Milky Way
Structure of the Galaxy
The Galaxy has the following components:
The disk
A thin disk, diameter up to 50 kpc, thickness
~1 kpc or less. Spiral arms are embedded within the disk
The bulge
Central roughly spherical region, ~1 kpc diameter The halo
Large roughly spherical region of low density.
Possibly as much as 50 or 100 kpc in diameter
I- Galactic disk
• The Galactic disk appears as a band of light stretching across our night sky, a band known as the Milky Way. if we look in a direction away from the Galactic disk, relatively few stars lie in our field of view. However, if our line of sight happens to lie within the disk, we see so many stars that their light merges into a continuous blur.
• The disk an immense, circular, flattened region containing most of our Galaxy's luminous stars and interstellar matter.
• The disk, the main body of the Galaxy, has a diameter of some 120.000 LY and thickness 1000 LY.
• Population I stars, open clusters, and interstellar clouds of gas and dust inhabit the disk, the gas extending out farther than stars.
• The Sun reside a little above the disk at a distance of approximately 30.000 LY from the Galaxy’s center.
II- Galactic bulge
• The galactic bulge encases the central regions of the Galaxy, including the mysterious nucleus, the very heart of the Galaxy.
• The bulge is about 12.000 LY in diameter and 10.000 LY thick. It contains little gas and dust but billions old population I stars.
III- Galactic halo
• If we look up or down out of the disk, we look away from the dust and gas, and we can see out into the halo of our galaxy.
• The spherical halo encircles the nuclear bulge and the disk.
• Globular clusters, containing Population II stars, make up the most
obvious material in the halo, which has a diameter of some 300.000 LY.
We are in a bad position to observe the Galaxy’s structure, for we reside in its dusty disk
• Stars of different types inhabit each part of the Galaxy: young, metal-rich Population I stars in the disk; old, metal-rich Population I stars in the bulge;
and very old , metal poor Population II stars in the halo.
• We are in a bad position to observe the Galaxy’s structure, for we reside in its dusty disk
• We believe that the disk of the Galaxy contains a few spiral arms. A spiral arm extends for thousands of light years and contains many hot, young stars, gas and dust with a density higher than in the region between arms.
• The spiral arms contain most of the gas, dust and young stars in the Galaxy
• Optical astronomers, who try to find the distances to spiral arms, fined their view blocked by interstellar dust.
• Radio waves get though. Radio astronomers can pick up the centimeter and millimeter radio emission from clouds of gas that mark spiral arms, but they have more difficulty in determining distances than optical astronomers do.
• We infer that our Galaxy has two major arms exterior to the sun’s orbit and perhaps four arms exterior to it wound around the nucleus
