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Here's a Python script that extracts 30 astronomical terms starting with "D" and exports them to an Excel file:

```python
import pandas as pd

# Define a dictionary of astronomical terms starting with "D"
astronomical_terms = {
    "Dark matter": {
        "Vietnamese": "Vật chất tối",
        "Explanation": "A type of matter hypothesized to account for a large part of the total mass in the universe."
    },
    "Dark energy": {
        "Vietnamese": "Năng lượng tối",
        "Explanation": "A hypothetical form of energy that permeates all of space and accelerates the expansion of the universe."
    },
    "Dwarf planet": {
        "Vietnamese": "Hành tinh lùn",
        "Explanation": "A small planet that does not clear its neighboring region of other objects."
    },
    "Dwarf star": {
        "Vietnamese": "Sao lùn",
        "Explanation": "A small star that is less luminous than main sequence stars."
    },
    "Dark nebula": {
        "Vietnamese": "Mây tối",
        "Explanation": "A cloud of gas and dust in space causing darkness not due to a hot gas cloud."
    },
    "Day": {
        "Vietnamese": "Ngày",
        "Explanation": "The time it takes for Earth to rotate once around its axis, approximately 24 hours."
    },
    "Delta-v": {
        "Vietnamese": "Delta-v",
        "Explanation": "The change in velocity of a spacecraft needed to perform a maneuver or reach orbit."
    },
    "Dark adaptation": {
        "Vietnamese": "Điều chỉnh ánh sáng tối",
        "Explanation": "The process by which the eye adjusts to low light levels, necessary for observing dark sky objects."
    },
    "Dwarf galaxy": {
        "Vietnamese": "Thiên hà lùn",
        "Explanation": "A galaxy of smaller size and fewer stars compared to mainstream galaxies."
    },
    "Dawn": {
        "Vietnamese": "Bình minh",
        "Explanation": "The time when the Sun begins to rise from the viewpoint of an observer."
    },
    "Deceleration parameter": {
        "Vietnamese": "Tham số giảm tốc",
        "Explanation": "A measure of the rate at which the expansion of the universe is slowing down."
    },
    "Diffuse nebula": {
        "Vietnamese": "Mây sao mờ",
        "Explanation": "A cloud of interstellar gas and dust, often the site of star formation."
    },
    "Dark side of the Moon": {
        "Vietnamese": "Mặt trăng hắc ám",
        "Explanation": "The hemisphere of the Moon that is not visible from Earth due to synchronous rotation."
    },
    "Double star": {
        "Vietnamese": "Sao đôi",
        "Explanation": "A pair of stars that appear close to each other in the sky."
    },
    "Doppler effect": {
        "Vietnamese": "Hiệu ứng Doppler",
        "Explanation": "An increase or decrease in the frequency of sound, light, or other waves as the source and observer move towards or away from each other."
    },
    "Dynamical time": {
        "Vietnamese": "Thời gian động lực",
        "Explanation": "A time scale used in celestial mechanics to predict the positions of celestial objects."
    },
    "Disk galaxy": {
        "Vietnamese": "Thiên hà đĩa",
        "Explanation": "A galaxy characterized by a flat, rotating disk of stars, gas, and dust."
    },
    "Dark adaptation": {
        "Vietnamese": "Điều chỉnh ánh sáng tối",
        "Explanation": "The process by which the eye adjusts to low light levels, necessary for observing dark sky objects."
    },
    "Declination": {
        "Vietnamese": "Sao khấu",
        "Explanation": "The angular distance of a celestial object north or south of the celestial equator."
    },
    "Density wave theory": {
        "Vietnamese": "Lý thuyết sóng mật độ",
        "Explanation": "A theory describing spiral arm structure in galaxies as waves of increased density."
    },
    "Deep space": {
        "Vietnamese": "Không gian sâu",
        "Explanation": "The vast, empty regions of space beyond the Earth's atmosphere."
    },
    "Differential rotation": {
        "Vietnamese": "Xoay chuyển biệt phân",
        "Explanation": "Rotation of a star or planet at different rates at different latitudes or depths."
    },
    "Diurnal motion": {
        "Vietnamese": "Chuyển động ngày",
        "Explanation": "The apparent daily motion of celestial objects across the sky due to the rotation of the Earth."
    },
    "Double cluster": {
        "Vietnamese": "Cụm đôi",
        "Explanation": "A pair of star clusters located close together in the sky."
    },
    "Dust lane": {
        "Vietnamese": "Vệt bụi",
        "Explanation": "A dark band of interstellar dust observed in many spiral galaxies."
    },
    "Dynamo theory": {
        "Vietnamese": "Lý thuyết động cơ",
        "Explanation": "A theory explaining the generation of magnetic fields in celestial bodies through fluid motion."
    },
    "Dark region": {
        "Vietnamese": "Vùng tối",
        "Explanation": "An area of the sky where there are fewer stars and nebulae, often indicating a region of dust or gas."
    },
    "Doppler shift": {
        "Vietnamese": "Dịch chuyển Doppler",
        "Explanation": "The change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source."
    },
    "Dust disk": {
        "Vietnamese": "Đĩa bụi",
        "Explanation": "A flat, rotating disk of dust and debris around a star or a young planetary system."
    },
    "Dark matter halo": {
        "Vietnamese": "Vòng tối",
        "Explanation": "A hypothetical component of a galaxy or galaxy cluster that envelops the visible parts."
    },
    "Dark cloud": {
        "Vietnamese": "Mây tối",
        "Explanation": "An interstellar cloud that is dense enough to obscure the light from stars behind it."
    },
    "Differential rotation": {
        "Vietnamese": "Sự xoay biệt phân",
        "Explanation": "The rotation of a celestial body where different parts rotate at different rates."
    },
    "Degenerate matter": {
        "Vietnamese": "Chất thoái hóa",
        "Explanation": "A state of matter under extreme pressure where quantum degeneracy pressure supports the object against gravitational collapse."
    },
    "Direct motion": {
        "Vietnamese": "Chuyển động trực tiếp",
        "Explanation": "The normal eastward motion of celestial bodies across the sky due to the rotation of the Earth."
    },
    "Dipole anisotropy": {
        "Vietnamese": "Độ lệch cực",
        "Explanation": "An uneven distribution of radiation in the Cosmic Microwave Background (CMB) that appears as a dipole pattern."
    }
}

# Convert dictionary to DataFrame
df = pd.DataFrame.from_dict(astronomical_terms, orient='index')
df.index.name = 'English Term'  # Set index name for Excel

# Specify the Excel file name
excel_file = "astronomical_terms_starting_with_D.xlsx"

# Export DataFrame to Excel
df.to_excel(excel_file, engine='openpyxl')

print(f"Excel file '{excel_file}' has been created successfully with 30 astronomical terms starting with 'D'.")
```

### Explanation of the Code:
- **Dictionary of Terms**: Contains 30 astronomical terms starting with "D", each with its Vietnamese translation and explanation.
- **DataFrame Creation**: Converts the `astronomical_terms` dictionary into a Pandas DataFrame (`df`) using `pd.DataFrame.from_dict()`.
- **Index Naming**: Sets the index name of the DataFrame to `'English Term'`.
- **Export to Excel**: Uses `df.to_excel()` to export the DataFrame to an Excel file named `"astronomical_terms_starting_with_D.xlsx"` using the `openpyxl` engine.
- **Confirmation**: Prints a message confirming the successful creation of the Excel file.

### Running the Code:
1. Copy the code into a Python script file (e.g., `export_astronomical_terms.py`).
2. Run the script using Python (`python export_astronomical_terms.py`).
3. Check your working directory for the generated Excel file (`astronomical_terms_starting_with_D.xlsx`).

This script will create an Excel file containing 30 astronomical terms starting with "D", along with their Vietnamese translations and explanations. Adjust the `astronomical_terms` dictionary to include additional terms or modify existing ones as needed.